Protobuf from pre-release to 3.12.1

[dwsutherland]

This is a small change with no associated Issue.

Change from Protobuf pre-release #3594 to 3.12.1.
Compile with new resulted in no change:

(flow) sutherlander@cortex-vbox:flow$ protoc --experimental_allow_proto3_optional -I=./ --python_out=./ data_messages.proto
(flow) sutherlander@cortex-vbox:flow$ git status
On branch protobuf-3.12.1
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   ../../setup.py

no changes added to commit (use "git add" and/or "git commit -a")
(flow) sutherlander@cortex-vbox:flow$ protoc --version
libprotoc 3.12.1

One review will probably do.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Already covered by existing tests.
• No change log entry required.
• No documentation update required.

[kinow]

CI happy, LGTM

[kinow]

Leaving it up to @oliver-sanders or @hjoliver to do the final review/merge.

[dwsutherland]

Leaving it up to @oliver-sanders or @hjoliver to do the final review/merge.

A one liner, and all tests passed.. But sure, no harm in waiting (not like alpha2 is going in tonight)..