Use patch file on Windows never executes mutants

[FlTr]

Using the option --use-patch-file on Windows is not possible.
Patch file and the resulting Config.covered_lines_by_filename array do use unix paths separators "/". While the Context.filename uses windows paths separators "\".

This causes should_exclude always to return True, and therefore no mutants are tested at all.

Config.covered_lines_by_filename = {
  'src/my-pkg/foo.py': { 1, 2, 42 }
}
Context.filename = 'src\\my-pkg\\foo.py'