pw_protobuf: update to use typing module for Set

I'm not a python expert, but our build was failing due to python being
confused about the syntax "set[str]". I'm guessing support for this was
added in some later version of python, but ours (3.7) doesn't seem to
support it, so this change updates it to import Set from the typing
module and use that instead.

Change-Id: I59435bab9375f70adb1053eb6709f3cea1336cd8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/100720
Commit-Queue: Aaron Okano <aaronokano@google.com>
Reviewed-by: Ted Pudlik <tpudlik@google.com>

pw_protobuf/py/pw_protobuf/symbol_name_mapping.py

@@ -14,13 +14,15 @@
 # the License.
 """Fixes identifiers that would cause compiler errors in generated C++ code."""
 
+from typing import Set
+
 # Set of words that can't be used as identifiers in the generated code. Many of
 # these are valid identifiers in proto syntax, but they need special handling in
 # the generated C++ code.
 #
 # Note: This is primarily used for "if x in y" operations, hence the use of a
 # set rather than a list.
-PW_PROTO_CODEGEN_RESERVED_WORDS: set[str] = {
+PW_PROTO_CODEGEN_RESERVED_WORDS: Set[str] = {
     # Identifiers that conflict with the codegen internals when used in certain
     # contexts:
     "Fields",