Upgrade pip in WIN CI

The windows CI is currently flaky: some dependencies like aiobotocore, multiprocess and seqeval sometimes fail to install.
In particular it seems that building the wheels fail. Here is an example of logs

```
Building wheel for seqeval (setup.py): started
  Running command 'C:\tools\miniconda3\envs\py37\python.exe' -u -c 'import io, os, sys, setuptools, tokenize; sys.argv[0] = '"'"'C:\\Users\\circleci\\AppData\\Local\\Temp\\pip-install-h55pfgbv\\seqeval_d6cdb9d23ff6490b98b6c4bcaecb516e\\setup.py'"'"'; __file__='"'"'C:\\Users\\circleci\\AppData\\Local\\Temp\\pip-install-h55pfgbv\\seqeval_d6cdb9d23ff6490b98b6c4bcaecb516e\\setup.py'"'"';f = getattr(tokenize, '"'"'open'"'"', open)(__file__) if os.path.exists(__file__) else io.StringIO('"'"'from setuptools import setup; setup()'"'"');code = f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' bdist_wheel -d 'C:\Users\circleci\AppData\Local\Temp\pip-wheel-x3cc8ym6'
  No parent package detected, impossible to derive `name`
  running bdist_wheel
  running build
  running build_py
  package init file 'seqeval\__init__.py' not found (or not a regular file)
  package init file 'seqeval\metrics\__init__.py' not found (or not a regular file)
  C:\tools\miniconda3\envs\py37\lib\site-packages\setuptools\command\install.py:37: SetuptoolsDeprecationWarning: setup.py install is deprecated. Use build and pip and other standards-based tools.
    setuptools.SetuptoolsDeprecationWarning,
  installing to build\bdist.win-amd64\wheel
  running install
  running install_lib
  warning: install_lib: 'build\lib' does not exist -- no Python modules to install

  running install_egg_info
  running egg_info
  creating UNKNOWN.egg-info
  writing UNKNOWN.egg-info\PKG-INFO
  writing dependency_links to UNKNOWN.egg-info\dependency_links.txt
  writing top-level names to UNKNOWN.egg-info\top_level.txt
  writing manifest file 'UNKNOWN.egg-info\SOURCES.txt'
  reading manifest file 'UNKNOWN.egg-info\SOURCES.txt'
  writing manifest file 'UNKNOWN.egg-info\SOURCES.txt'
  Copying UNKNOWN.egg-info to build\bdist.win-amd64\wheel\.\UNKNOWN-0.0.0-py3.7.egg-info
  running install_scripts
  creating build\bdist.win-amd64\wheel\UNKNOWN-0.0.0.dist-info\WHEEL
  creating 'C:\Users\circleci\AppData\Local\Temp\pip-wheel-x3cc8ym6\UNKNOWN-0.0.0-py3-none-any.whl' and adding 'build\bdist.win-amd64\wheel' to it
  adding 'UNKNOWN-0.0.0.dist-info/METADATA'
  adding 'UNKNOWN-0.0.0.dist-info/WHEEL'
  adding 'UNKNOWN-0.0.0.dist-info/top_level.txt'
  adding 'UNKNOWN-0.0.0.dist-info/RECORD'
  removing build\bdist.win-amd64\wheel
  Building wheel for seqeval (setup.py): finished with status 'done'
  Created wheel for seqeval: filename=UNKNOWN-0.0.0-py3-none-any.whl size=963 sha256=67eb93a6e1ff4796c5882a13f9fa25bb0d3d103796e2525f9cecf3b2ef26d4b1
  Stored in directory: c:\users\circleci\appdata\local\pip\cache\wheels\05\96\ee\7cac4e74f3b19e3158dce26a20a1c86b3533c43ec72a549fd7
  WARNING: Built wheel for seqeval is invalid: Wheel has unexpected file name: expected 'seqeval', got 'UNKNOWN'
```

I tried to update pip and re-run the CI several times and I couldn't re-experience this issue for now, so I think upgrading pip may solve the issue