Pylint-protobuf

- Add support for scalar repeated fields
- Fix false-negative warnings on some implicitly defined protobuf message
fields (e.g. ParseFromString is not defined on Enums, nor is Value defined on
messages)
- Add partial support for composite repeated fields (only nested messages
currently supported)
- Fix integration with default typecheck checker (E1101 no-member warnings) on
protobuf well-known types (37). Thanks mabrowning for the PR.

- Allow warnings on unknown keyword arguments in protobuf message constructors
(originally suppressed to fix 30)
- Fix incorrect AST transform of deeply-nested message types (33, 34)
- Add better support for map fields (35)
- Change handling of inferred values around ambiguous cases where a name may be
a protobuf message (36)

Thanks to mabrowning for the reports and PRs! (33, 34, 35, 36)

- Fix spurious warnings on use of keyword arguments in protobuf message
constructors due to integration with pylint typecheck (30)
- Fix some false-positives on protobuf enum types (31)
- Fix spurious warnings on protobuf map fields due to integration with pylint
typecheck (32). Thanks to mabrowning for these reports (30, 31, 32).

- Warning and suppression of ImportError for Python 3.9 and protobuf 3.14

- Rewrite of the internals of the checker to compare usages against protobuf
message and field descriptors. This change represents a significant departure
in the design of the checker towards an AST transform of pb2 modules into
plain class definitions and leveraging the astroid inference framework to
track scope changes and name shadowing. This release should have feature
parity with 0.13 (some message details have changed, though message codes
remain the same), please raise an issue if behaviours have changed in
unexpected ways.
- This release should address a number of outstanding issues around nested
message and enum definitions (16, 21, 24), renaming/aliasing (23),
and dynamic typing (26). Thanks diana-infinitus-ai, fivepapertigers,
mishas, NickeZ, sagar-infinitus-ai, and xSAVIKx for the reports.

- Fix false positive warnings on nested enum definitions. Thanks
diana-infinitus-ai for the PR (28)