Include flair/py.typed and requirements.txt in source distribution

[dobbersc]

This PR fixes the following issues by including the flair/py.typed and requirements.txt in the source distribution (sdist):

• The py.typed file was added in add py.typed file for PEP-561 compatibility #2858. Although the file was added to the repository, it is not present in the distributed package.
• The requirements.txt file was not included in flair-0.12.2.tar.gz on PyPI, which was causing issues when installing flair via the tarball. Closes requirements.txt missing from PyPI source tarball #2759

This is done by including these files in the source distribution (sdist) by adding a MANIFEST.in file. To test that this works, git checkout this branch, and run:

python setup.py sdist

Then untar the flair-0.12.2.tar.gz file. You should see the requirements.txt and py.typed files inside. Confirm that things work by running the following command:

pip install flair-0.12.2.tar.gz

This should install the package and report:

Processing ./flair-0.12.2.tar.gz
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
  Preparing metadata (pyproject.toml) ... done
[...]
Successfully built flair
Installing collected packages: flair
Successfully installed flair-0.12.2

[alanakbik]

@dobbersc thanks for fixing this!