remove internal self.fill, and keep padding_mode as string or number

[romainVala]

Fixes #376.

Description

I remove internal fill attribute, to keep padding_mode as given in input (a string, or a number for the constant mode)
parse_padding_mode was then only use to check padding_mode strings, I rename it to
check_padding_mode

Checklist

• I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• Non-breaking change (would not break existing functionality)
• Breaking change (would cause existing functionality to change)
• Tests added or modified to cover the changes
• Integration tests passed locally by running pytest
• In-line docstrings updated
• Documentation updated, tested running make html inside the docs/ folder
• This pull request is ready to be reviewed
• If the PR is ready and there are multiple commits, I have squashed them and force-pushed

[fepegar]

Awesome. I added a test that fails on the master branch. Thanks, @romainVala!

[romainVala]

great, first try was good !
happy to participate