Mypy-protobuf

- Organized generated enum code to prevent definition ordering issues in Pyright-based linters
- Changed type generation for `grpcio` stubs to use the `MultiCallable` API ([see here](https://grpc.github.io/grpc/python/grpc.html#multi-callable-interfaces)) . This requires using the `grpc-stubs` typings for grpcio. This change should allow calling stub methods with common parameters (`timeout`, `metadata`, etc.) as well as calling methods on the `MultiCallable` object (e.g. `my_stub.MyRpcMethod.future()`).
- Update stubs to mark repeated scalar and repeated enum fields as read-only

- Add support for `_FIELD_NUMBER` generated fields on messages as specified [in the spec](https://developers.google.com/protocol-buffers/docs/reference/python-generated#fields)

- Fix CI caching across version updates.
- Added unit testing for EnumTypeWrapper.Value usage
- Reexport of top level enum values when using `import public`

- Fix bug where module level `DESCRIPTOR: FileDescriptor` was not being generated for files w public imports

- Fix crash when a import public reexport when dependent proto file is not listed on command line for generation

Non Backward Compatible Changes

- Dropping support for running mypy-protobuf in python <= 3.5. Note you can still generate stubs target-compatible to python2
- Type proto Enum values for as `MyEnum.V` rather than `MyEnumValue` for import ergonomics,
allowing the caller to import `MyEnum` rather than conditionally importing `MyEnumValue`
- Default disallow `None` as argument for primitive fields of constructors in proto3.
Provided `relax_strict_optional_primitives` flag to relax this strictness if you prefer.

New Features

- Support for `grpcio` stubs generation
- Allow `mypy_protobuf.py` to be run directly as a script
- Add support for proto's [`well_known_types`](https://developers.google.com/protocol-buffers/docs/reference/python-generated#wkt)
- Support message fields named `self` - by renaming the constructor's `self` to `self_`
- Rename extensions proto from `mypy/mypy.proto` to `mypy_protobuf/extensions.proto`
- Add support for mypy-proto extensions `mypy_protobuf.casttype`, `mypy_protobuf.keytype`, and `mypy_protobuf.valuetype`
- Add support for `import public` proto imports - by reexporting in generated code

Output Format

- Generate fully qualified references rather than mangling
- Import builtins library rather than mangling builtins
- Use fully qualified names rather than mangling imports
- Only mangle-alias top-level identifiers w/ `global___` to avoid conflict w/ fields of same name [previously was mangling inner messages as well]
- Add support for `readable_stubs` parameter for mangle-free output code w/o fully-qualified references to message.
(in many cases this is good enough and easier to read)
- Generate `arg: Optional[type] = ...` instead of `arg: Optional[type] = None`
- Avoid importing google.protobuf.message.Message unless it's needed

Internal Improvements

- Add support for python 3.9 to CI
- Update mypy-protobuf CI to target 3.8 rather than 3.5
- Inline mypy annotations, eliminate six, and remove `__future__` import in `mypy_protobuf_lib.py`
- Flatten directory structure (remove python subdirectory). Updated unreleased installation instructions.
- Deprecate and delete the go/ implementation