Grpc-interceptor

Added
- Run tests on Python 3.9 and default to that for linting, etc.

Fixed
- Raise UNIMPLEMENTED instead of UNKNOWN when no handler is found for a method (thanks Richard Mahlberg!)

Added
- Client-side interceptors (thanks Michael Morgan!)

Changed (breaking)
- Added a `context` parameter to special case functions in the `testing` module

Fixed
- Build issue caused by [pip upgrade](https://github.com/cjolowicz/hypermodern-python/issues/174#issuecomment-745364836)
- Docs not building correctly in nox

Added
- Support for all streaming RPCs

Added
- Expose some imports from the top-level

Changed (breaking)
- Rename to `ServerInterceptor` (do not intend to make breaking name changes after this)

Added
- `status_on_unknown_exception` to `ExceptionToStatusInterceptor`
- `py.typed` (so `mypy` will type check the package)

Fixed
- Allow protobuf version 4.0.x which is coming out soon and is backwards compatible
- Testing in autodocs
- Turn on xdoctest
- Prevent autodoc from outputting default namedtuple docs

Changed (breaking)
- Rename `Interceptor` to `ServiceInterceptor`

Added
- The `testing` module
- Some helper functions
- Improved test coverage

Fixed
- Protobuf compatibility improvements