Switch to getting source from pypi

[xylar]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[xylar]

@conda-forge-admin, please rerender

[xylar]

A local CPython and PyPy test both worked so I'm hopeful that this will work here, too.

[xylar]

@conda-forge/protobuf, I am pretty new as a maintainer here. Do any of you know if there's a good reason we haven't been using source from PyPI?

It initially seemed that this might help us with #167. However, it sounds like that isn't the case, see protocolbuffers/protobuf#10305 (comment)

[xylar]

I don't think this is going to solve any problems so let's not do this.