Warn, instead of error, when Python targets have non-Python sources (#9752)
Warn, instead of error, when Python targets have non-Python sources (#9752)

In my excitement to demo the new features of the Target API, I made a breaking change without following the proper deprecation cycle. 

It's true that Python files should only depend on `.py` files, but we need to do a deprecation cycle for this.

[ci skip-rust-tests]
[ci skip-jvm-tests]
pantsbuild/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,jsirois/pants,jsirois/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,jsirois/pants,pantsbuild/pants,benjyw/pants,benjyw/pants