Provide an alternative to using subprocess.run patch

[MaxOfLondon]

Description

It would be nice to have a fallback to patch if subprocess is not supported on the platform like for example emscripten. An alternative could be https://github.com/conan-io/python-patch-ng.git but I am sure other also exist and could be evaluated. Thank you in advance!

[adamchainz]

Yeah, I've always wanted to use a pure Python in-memory patch library. When I wrote patchy, none seemed easy. Perhaps python-patch-ng is the one, idk. If you'd like to put together a proof-of-concept that would be neat.