This PR is adding requirements.txt filename into MANIFEST.in to fix the following error I'm getting when installing the library:

Traceback (most recent call last):
File "setup.py", line 21, in
with open('requirements.txt') as f: # pylint: disable=W1514
FileNotFoundError: [Errno 2] No such file or directory: 'requirements.txt'

Checklist

General Contributing

• [X ] Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• [X ] SDK Code Change
• Example/Test Code Change

Validation

• [X ] Does tox pass?
• Do the tests pass?