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 Job ID Name Coverage
  Preparation
passed #81819
gst indent

00:00:16

passed #81818
manifest

00:00:35

 
  Build
passed #81820
build fedora x86_64

00:01:44

 
  Test
passed #81821
check fedora

00:03:34

 
  Full Builds
passed #81822
build android arm64 api28

00:05:32

passed #81824
build cerbero cross win32

00:09:30

passed #81825
build cerbero cross win64

00:09:41

passed #81823
build cerbero fedora x86_64

00:09:54