Add flakehell python linter (#3295)

[a666]

Adds flakehell.

For the moment it is just a blatant copy of the flake8 linter.

[a666]

This seemed to make more sense that appending the module in the options.

[hsanson]

Impressive PR, thanks. One test on windows is failing, once that is fixed it looks good to merge:

    ( 7/17) [EXECUTE] (X) ['flakehell', ['flakehell --version', 'flakehell lint --format=default --stdin-display-name %s -']] should be equal to ['C:\testplugin\test\test-files\python\with_virtualenv\env\Scripts\flakehell', ['C:\testplugin\test\test-files\python\with_virtualenv\env\Scripts\flakehell --version', 'C:\testplugin\test\test-files\python\with_virtualenv\env\Scripts\flakehell lint --format=default --stdin-display-name %s -']]

[a666]

Thanks.
I'm unsure about how to fix that test, it is on parity with the (non-failing) flake8 test:

-Execute(The flake8 callbacks should detect virtualenv directories):
+Execute(The flakehell callbacks should detect virtualenv directories):
   silent execute 'file ' . fnameescape(g:dir . '/../test-files/python/with_virtualenv/subdir/foo/bar.py')

   let b:executable = ale#path#Simplify(
-  \ g:dir . '/../test-files/python/with_virtualenv/env/' . b:bin_dir . '/flake8'
+  \ g:dir . '/../test-files/python/with_virtualenv/env/' . b:bin_dir . '/flakehell'
   \)

   AssertLinter b:executable, [
   \ ale#Escape(b:executable) . ' --version',
   \ ale#Escape(b:executable)
+  \   . ' lint'
   \   . ' --format=default'
   \   . ' --stdin-display-name %s -',
   \]

And I don't see anything in the code that's done any differently with respect to flake8.
Am I missing something about windows?

[a666]

Final test now passes.

[hsanson]

Thanks for the contribution.