flake8ify

Made some stuff flake8-compliant.

Before:

```
❯ tox -e flake8
GLOB sdist-make: /Users/marca/dev/git-repos/Python-Markdown/setup.py
flake8 inst-nodeps: /Users/marca/dev/git-repos/Python-Markdown/.tox/dist/Markdown-2.6.dev.zip
flake8 runtests: PYTHONHASHSEED='1581807905'
flake8 runtests: commands[0] | flake8 /Users/marca/dev/git-repos/Python-Markdown/markdown /Users/marca/dev/git-repos/Python-Markdown/tests /Users/marca/dev/git-repos/Python-Markdown/setup.py /Users/marca/dev/git-repos/Python-Markdown/run-tests.py
/Users/marca/dev/git-repos/Python-Markdown/markdown/util.py:18:19: F821 undefined name 'basestring'
/Users/marca/dev/git-repos/Python-Markdown/markdown/util.py:19:17: F821 undefined name 'unicode'
/Users/marca/dev/git-repos/Python-Markdown/markdown/util.py:20:15: F821 undefined name 'unichr'
/Users/marca/dev/git-repos/Python-Markdown/setup.py:62:22: E211 whitespace before '('
/Users/marca/dev/git-repos/Python-Markdown/setup.py:65:22: E211 whitespace before '('
/Users/marca/dev/git-repos/Python-Markdown/setup.py:150:18: E211 whitespace before '('
/Users/marca/dev/git-repos/Python-Markdown/setup.py:177:34: E211 whitespace before '('
ERROR: InvocationError: '/Users/marca/dev/git-repos/Python-Markdown/.tox/flake8/bin/flake8 /Users/marca/dev/git-repos/Python-Markdown/markdown /Users/marca/dev/git-repos/Python-Markdown/tests /Users/marca/dev/git-repos/Python-Markdown/setup.py /Users/marca/dev/git-repos/Python-Markdown/run-tests.py'
___________________________________________________________________________________ summary ____________________________________________________________________________________
ERROR:   flake8: commands failed
```

After:

```
❯ tox -e flake8
GLOB sdist-make: /Users/marca/dev/git-repos/Python-Markdown/setup.py
flake8 inst-nodeps: /Users/marca/dev/git-repos/Python-Markdown/.tox/dist/Markdown-2.6.dev.zip
flake8 runtests: PYTHONHASHSEED='3348188633'
flake8 runtests: commands[0] | flake8 /Users/marca/dev/git-repos/Python-Markdown/markdown /Users/marca/dev/git-repos/Python-Markdown/tests /Users/marca/dev/git-repos/Python-Markdown/setup.py /Users/marca/dev/git-repos/Python-Markdown/run-tests.py
___________________________________________________________________________________ summary ____________________________________________________________________________________
  flake8: commands succeeded
  congratulations :)
```