Change Python project root dir detection for flake8 configuration

The official configuration files for `flake8` are `.flake8`, `tox.ini`,
and `setup.cfg`.

After investigation, it is safe to remove `flake8.cfg` as it appears to
only exist as a typo in other tooling documentation (e.g.,
`python-language-server`).

Even though no linters automatically read `.flake8rc`, it is kept in
case projects may be using it for detecting the projects root directory.

autoload/ale/python.vim

@@ -25,7 +25,7 @@ function! ale#python#FindProjectRootIni(buffer) abort
         \|| filereadable(l:path . '/tox.ini')
         \|| filereadable(l:path . '/mypy.ini')
         \|| filereadable(l:path . '/pycodestyle.cfg')
-        \|| filereadable(l:path . '/flake8.cfg')
+        \|| filereadable(l:path . '/.flake8')
         \|| filereadable(l:path . '/.flake8rc')
         \|| filereadable(l:path . '/pylama.ini')
         \|| filereadable(l:path . '/pylintrc')

doc/ale-python.txt

@@ -29,7 +29,7 @@ ALE will look for configuration files with the following filenames. >
   tox.ini
   mypy.ini
   pycodestyle.cfg
-  flake8.cfg
+  .flake8
   .flake8rc
   pylama.ini
   pylintrc