Pylint-protobuf

- Fix false positive protobuf-type-error on numeric scalar fields supporting
type elevation, e.g. int to float (41)
- Fix false positive protobuf-type-error on keyword arguments supplied with
default None values (42)
- Fix false positive protobuf-type-error on partially inferred values assigned
to scalar fields (40). Thanks zoedberg for the report

- Fix IndexError raised in `_check_repeated_scalar` (39). Thanks zoedberg for
the report

- Fix AttributeError raised in `_check_repeated_scalar`

- Fix AttributeError raised in `_check_init_kwargs` method (38). Thanks
mcharlou for the report

- Fix missing .append() (and other) methods on repeated composite containers
causing false positive warnings

- Add support for checking invalid enum values and names. This introduces a new
warning message "protobuf-enum-value"
- Add support for type checking scalar fields. This introduces a new warning
message "protobuf-type-error"
- Add warning for use of positional arguments on message constructors. This
introduces a new warning message "protobuf-no-posargs"
- Add support for type checking message constructor arguments
- Add support for type checking repeated scalar fields
- Add support for checking assignment to read-only fields. This introduces a
new warning message "protobuf-no-assignment"
- Fix missing warnings for non-nested composite types and external nested
(e.g. External.Inner) composite types