flakehell install failures

Both @alex1126 and I are running into an issue where flakehell is failing to install while setting up the pre-commit hooks documented in the [contributor guide](https://meltano.com/docs/contributor-guide.html#setting-up-your-environment):

```shell
(melty-3.8.12) ➜  meltano git:(master) poetry run pre-commit install --install-hooks
pre-commit installed at .git/hooks/pre-commit
[INFO] Installing environment for https://github.com/life4/flakehell.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
An unexpected error has occurred: CalledProcessError: command: ('/Users/syn/.cache/pre-commit/repo72kapjci/py_env-python3.8/bin/python', '-mpip', 'install', '.')
return code: 1
expected return code: 0
stdout:
    Processing /Users/syn/.cache/pre-commit/repo72kapjci
      Installing build dependencies: started
      Installing build dependencies: finished with status 'done'
      Getting requirements to build wheel: started
      Getting requirements to build wheel: finished with status 'done'
      Installing backend dependencies: started
      Installing backend dependencies: finished with status 'done'
      Preparing metadata (pyproject.toml): started
      Preparing metadata (pyproject.toml): finished with status 'error'

stderr:
      ERROR: Command errored out with exit status 1:
       command: /Users/syn/.cache/pre-commit/repo72kapjci/py_env-python3.8/bin/python /Users/syn/.cache/pre-commit/repo72kapjci/py_env-python3.8/lib/python3.8/site-packages/pip/_vendor/pep517/in_process/_in_process.py prepare_metadata_for_build_wheel /var/folders/47/sydnn1xj5vv6krsn8n4krgjw0000gn/T/tmpet2njlov
           cwd: /Users/syn/.cache/pre-commit/repo72kapjci
      Complete output (36 lines):
      Traceback (most recent call last):
        File "/Users/syn/.cache/pre-commit/repo72kapjci/py_env-python3.8/lib/python3.8/site-packages/pip/_vendor/pep517/in_process/_in_process.py", line 363, in <module>
          main()
        File "/Users/syn/.cache/pre-commit/repo72kapjci/py_env-python3.8/lib/python3.8/site-packages/pip/_vendor/pep517/in_process/_in_process.py", line 345, in main
          json_out['return_val'] = hook(**hook_input['kwargs'])
        File "/Users/syn/.cache/pre-commit/repo72kapjci/py_env-python3.8/lib/python3.8/site-packages/pip/_vendor/pep517/in_process/_in_process.py", line 164, in prepare_metadata_for_build_wheel
          return hook(metadata_directory, config_settings)
        File "/private/var/folders/47/sydnn1xj5vv6krsn8n4krgjw0000gn/T/pip-build-env-znhipti1/overlay/lib/python3.8/site-packages/flit_core/buildapi.py", line 49, in prepare_metadata_for_build_wheel
          metadata = make_metadata(module, ini_info)
        File "/private/var/folders/47/sydnn1xj5vv6krsn8n4krgjw0000gn/T/pip-build-env-znhipti1/overlay/lib/python3.8/site-packages/flit_core/common.py", line 396, in make_metadata
          md_dict.update(get_info_from_module(module, ini_info.dynamic_metadata))
        File "/private/var/folders/47/sydnn1xj5vv6krsn8n4krgjw0000gn/T/pip-build-env-znhipti1/overlay/lib/python3.8/site-packages/flit_core/common.py", line 193, in get_info_from_module
          docstring, version = get_docstring_and_version_via_import(target)
        File "/private/var/folders/47/sydnn1xj5vv6krsn8n4krgjw0000gn/T/pip-build-env-znhipti1/overlay/lib/python3.8/site-packages/flit_core/common.py", line 169, in get_docstring_and_version_via_import
          m = sl.load_module()
        File "<frozen importlib._bootstrap_external>", line 522, in _check_name_wrapper
        File "<frozen importlib._bootstrap_external>", line 1022, in load_module
        File "<frozen importlib._bootstrap_external>", line 847, in load_module
        File "<frozen importlib._bootstrap>", line 265, in _load_module_shim
        File "<frozen importlib._bootstrap>", line 702, in _load
        File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
        File "<frozen importlib._bootstrap_external>", line 843, in exec_module
        File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
        File "/Users/syn/.cache/pre-commit/repo72kapjci/flakehell/__init__.py", line 5, in <module>
          from ._cli import entrypoint, flake8_entrypoint
        File "/Users/syn/.cache/pre-commit/repo72kapjci/flakehell/_cli.py", line 9, in <module>
          from .commands import COMMANDS
        File "/Users/syn/.cache/pre-commit/repo72kapjci/flakehell/commands/__init__.py", line 5, in <module>
          from ._baseline import baseline_command
        File "/Users/syn/.cache/pre-commit/repo72kapjci/flakehell/commands/_baseline.py", line 6, in <module>
          from .._patched import FlakeHellApplication
        File "/Users/syn/.cache/pre-commit/repo72kapjci/flakehell/_patched/__init__.py", line 2, in <module>
          from ._app import FlakeHellApplication
        File "/Users/syn/.cache/pre-commit/repo72kapjci/flakehell/_patched/_app.py", line 10, in <module>
          from flake8.options.config import MergedConfigParser, get_local_plugins
      ImportError: cannot import name 'MergedConfigParser' from 'flake8.options.config' (/private/var/folders/47/sydnn1xj5vv6krsn8n4krgjw0000gn/T/pip-build-env-znhipti1/normal/lib/python3.8/site-packages/flake8/options/config.py)
```

This seems to be related to https://github.com/flakehell/flakehell/issues/22 but the versions don't actually match up:

```
(melty-3.8.12) ➜  meltano git:(master) poetry run flakehell --version
FlakeHell 0.7.1
Flake8    3.9.2
For plugins versions use flakehell plugins
```

Digging some more, @edgarrmondragon found that pre-commit actually creates it own venv's and doesn't take into account pyproject.toml - and so the latest versions of flakehell/flake8 are installed - which then trigger https://github.com/flakehell/flakehell/issues/22.

Story points: 0.0