Migrate to protobuf-py and rename repo to connect-py

[anuraaga]

This brings the long awaited update to target the new protobuf-py implementation. Along with it, we will rename the repo to connect-py so this incorporates docs updates for that as well.

• Default dependency is protobuf-py
• A compat package with codecs targeting google.protobuf is included, and automatically used by gencode if passing the option protobuf=google
• Rename protoc-gen-connect-python to protoc-gen-connectrpc which could have happened before too
• Docs are generally updated to follow connect-es's pattern of using just Connect in prose, and connect-py only in repo links (before we used Connect-Python in prose)
• I will backfill unit tests for the codegen plugin (it is mostly tested implicitly except for the io option). I owe a testing utility for this in protobuf-py