fix: include requirements files in python sdist

[dhellmann]

Description of your changes:

The source distribution produced does not include all of the files
needed to rebuild the wheel. This change adds a MANIFEST.in file to
cause the requirements.txt and requirements-dev.txt files to be included
so when the setup.py tries to read them they are present.

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[google-oss-prow]

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[dhellmann]

@HumairAK it looks like you've modified the python packaging code most recently, could you take a look at this please? We're trying to rebuild the package from source for a downstream distribution and the missing files mean we can't build from the sdist. Thanks!

[HumairAK]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HumairAK

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