Update MANIFEST.in to include requirements.txt

[nikkobautista]

Summary

Add requirements.txt to make sure build via sdist will work.

Currently seeing this issue when rebuilding the wheel locally via pip install --no-binary :all: d20

    ERROR: Command errored out with exit status 1:
     command: /Library/Developer/CommandLineTools/usr/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/var/folders/98/mwzf553x5ys5zzgt2hqxn1pc000b16/T/pip-install-10kno646/d20_30803ea682c845c3bfa1b1efc5c18192/setup.py'"'"'; __file__='"'"'/private/var/folders/98/mwzf553x5ys5zzgt2hqxn1pc000b16/T/pip-install-10kno646/d20_30803ea682c845c3bfa1b1efc5c18192/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /private/var/folders/98/mwzf553x5ys5zzgt2hqxn1pc000b16/T/pip-pip-egg-info-ly28dsj6
         cwd: /private/var/folders/98/mwzf553x5ys5zzgt2hqxn1pc000b16/T/pip-install-10kno646/d20_30803ea682c845c3bfa1b1efc5c18192/
    Complete output (5 lines):
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/private/var/folders/98/mwzf553x5ys5zzgt2hqxn1pc000b16/T/pip-install-10kno646/d20_30803ea682c845c3bfa1b1efc5c18192/setup.py", line 6, in <module>
        with open("requirements.txt", "r") as fh:
    FileNotFoundError: [Errno 2] No such file or directory: 'requirements.txt'
    ----------------------------------------

Checklist

PR Type

• [ ] This PR is a code change that implements a feature request.
• [X] This PR fixes an issue.
• [ ] This PR adds a new feature that is not an open feature request.
• [X] This PR is not a code change (e.g. documentation, README, ...)

  Other

• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] If code changes were made then they have been tested.
• [ ] I have updated the documentation to reflect the changes.