fix: include package data

[korikuzma]

close #256 . Followed https://setuptools.pypa.io/en/latest/userguide/datafiles.html#subdirectory-for-data-files

Notes:

• Includes transcript_mappings.tsv

[jsstevenson]

I'm ok with pushing that failed docs build to a separate issue if you want to get this out now

[korikuzma]

@jsstevenson tbh, I do not know how the docs work

[jsstevenson]

@korikuzma same!

[korikuzma]

python3 -m pip install -i https://test.pypi.org/simple/ cool-seq-tool --no-deps