Remove all custom flake8 configuration

This is a new code base, so there's no need to ignore things. I was only
going to sync with Matplotlib's config, but none of the existing code
violated any of the additional ignores, and we only have those in
Matplotlib to prevent mass changes.

Also, we should definitely not ignore `__init__.py`, as that's the only
file with code in it.

Author: QuLogic

mpl_sphinx_theme/__init__.py

@@ -1,4 +1,4 @@
-from ._version import version_info, __version__
+from ._version import version_info, __version__  # noqa: F401
 
 from pathlib import Path
 
@@ -8,7 +8,8 @@ def get_html_theme_path():
     return [str(Path(__file__).parent.parent.resolve())]
 
 
-# See https://www.sphinx-doc.org/en/master/development/theming.html#distribute-your-theme-as-a-python-package
+# For more details, see:
+# https://www.sphinx-doc.org/en/master/development/theming.html#distribute-your-theme-as-a-python-package
 def setup(app):
     app.add_html_theme("mpl_sphinx_theme",
                        str(Path(__file__).parent.resolve()))

setup.cfg

@@ -3,20 +3,9 @@
 # https://flake8.readthedocs.io/en/latest/user/configuration.html
 # https://flake8.readthedocs.io/en/latest/user/error-codes.html
 
-# Aligned with black https://github.com/psf/black/blob/main/.flake8
-extend-ignore = E203, E266, E501
-# Note: there cannot be spaces after commas here
-exclude = __init__.py,versioneer.py
-ignore =
-    E4,         # Import formatting
-    E731,       # Assigning lambda expression
-    W503,       # line break before binary operator
-
-per-file-ignores =
-    **/tests/*:
-        # local variable is assigned to but never used
-        F841,
-        # Ambiguous variable name
-        E741,
-
-max-line-length = 88
+exclude =
+    .git
+    build
+    # External files.
+    .tox
+    .eggs