python: upgrade to protoc v31.0

[julieqiu]

We plan to use v31.0 for Go and Python (#4816) as they onboard to Librarian.

It would be useful to understand whether Python generation can work with protoc v31.0, and if not, what the specific constraints are.

The goal is to better understand the limitations and tradeoffs as we think about versioning more systematically in librarian. See #4815 for context.

[julieqiu]

/cc @jskeet

[jskeet]

I can check what happens with v31.0 tomorrow. I have memories of there being some problems which are in the middle of being resolved, but I'll know more tomorrow.

I would prefer to perform the migration with the currently-used version though, with a fast-follow to use v31.0, or to change the legacylibrarian generation to use v31.0 as well before migration, so that the actual migration can be done in a minimal way. (Basically I'd prefer not to mix changes that are inherent to the migration with changes that are due to changing versions.)

[jskeet]

TL;DR: We need to confirm everything with the Python team, and further changes will probably be needed, but I'm hoping it'll be reasonable to do this fairly soon after migration.

Okay, it definitely still generates - and most packages aren't affected, because Python only uses the built-in Python plugin for a few APIs.

But there are significant differences for proto_only API paths. A quick summary of differences I've seen so far:

• Addition of a comment "NO CHECKED-IN PROTOBUF GENCODE" which we'd probably want to remove
• Comment version change from "Protobuf Python Version: 4.25.3" to "Protobuf Python Version: 6.31.0"
• Addition of _runtime_version.ValidateProtobufRuntimeVersion call which I suspect will break things without other changes (i.e. we need to update the runtime dependency)
• Change from using "_options" to "_loaded_options"

To save me looking it up again, here are the proto_only API paths:

• google/identity/accesscontextmanager/v1
• google/identity/accesscontextmanager/type
• google/cloud/audit
• google/cloud/orgpolicy/v1
• google/api
• google/cloud
• google/rpc
• google/type
• google/cloud/location
• google/logging/type
• google/rpc/context
• google/iam/v1

[jskeet]

I'll move this to post-migration stabilization as it's not blocking migration.