pre-commit-python: Allow line breaks between binary operators

pre-commit-python overrides the list of ignored Python style errors.
Unfortunately, before this patch the list did not exclude W503 and
W504 (which are otherwise ignored by default).

The consequence of having those two warnings enabled at the same time is
that it's not possible to break lines on binary operators, which is an
unreasonable unintentional restriction:

  'validateflow': "validateflow, expectations-dir=\"" +
                  expectations_dir + "\", actual-results-dir=\"" +
                  actual_results_dir + "\"",

W504 line break after binary operator

  'validateflow': "validateflow, expectations-dir=\""
                  + expectations_dir + "\", actual-results-dir=\""
                  + actual_results_dir + "\"",

W503 line break before binary operator

This patch excludes W503 so that there is a valid style for breaking
lines on binary operators.
8 jobs for python-style-warnings in 15 minutes and 50 seconds (queued for 1 second)
Status Name Job ID Coverage
  Preparation
passed gst indent #81819

00:00:16

passed manifest #81818

00:00:35

 
  Build
passed build fedora x86_64 #81820

00:01:44

 
  Test
passed check fedora #81821

00:03:34

 
  Full Builds
passed build android arm64 api28 #81822

00:05:32

passed build cerbero cross win32 #81824

00:09:30

passed build cerbero cross win64 #81825

00:09:41

passed build cerbero fedora x86_64 #81823

00:09:54