Merge branch 'ab/perl-python-attrs'

We learned that our source files with ".pl" and ".py" extensions
are Perl and Python files respectively and changes to them are
better viewed as such with appropriate diff drivers.

* ab/perl-python-attrs:
  .gitattributes: add a diff driver for Python
  .gitattributes: use the "perl" differ for Perl
  .gitattributes: add *.pl extension for Perl