Loosen protocolbuffers/python protobuf runtime dependency (#2053)

We currently set a minimum version of the runtime `protobuf` package for
protocolbuffers/python based on the documented ["new gencode + old
runtime = never allowed"][1] rule that generally applies to the
protocolbuffers plugins - that the runtime versions are free to add
runtime APIs in any release and it's expected that the gencode must be
updated to run with the new runtime version.

However, [Python has a separate carve-out behavior for its gencode][2],
documented now as:

> Since the 3.20.0 release, the Protobuf Python generated code became a
> thin wrapper around an embedded FileDescriptorProto. Because these
> protos are supported on extremely long timeframes, our usual major
> version compatibility windows aren’t typically necessary.

> Python may break generated code compatibility in specific future major
> version releases, but it will be coupled with poison pill warnings and
> errors in advance. As of 6.32.0, all generated code since 3.20.0 will
be
> supported until at least 8.x.y.

This means that we don't need to pin the protocolbuffers/python version
to the matching runtime version; we can loosen the minimum to `3.20.0`
(and will need to reevaluate this in the future v8 major release).

This commit pins the latest minor release in each major version to a
minimum of 3.20.

An alternative is to _just_ specify `protobuf` without any version
specifiers, and allow consumers to pick the version of `protobuf` they
want to supply. But since the minimum is pretty clearly documented, I
think supplying it is reasonable.

[1]:
https://protobuf.dev/support/cross-version-runtime-guarantee/#backwards
[2]:
https://protobuf.dev/support/cross-version-runtime-guarantee/#python


---

* The [first
commit](8e4141a)
applies the above on the latest minor release of each major version of
protocolbuffers/python (and protocolbuffers/pyi, which
protocolbuffers/python depends on).
* The [second
commit](289a3e8)
does the rest of the versions. We can revert this commit if we'd like,
but I think it would be better to update all of these in one shot to
avoid confusion.
* The [third
commit](72a8bc5)
is optional, but in the same vein: we currently set a fairly strict
lower bound on the `types-protobuf` dependency of protocolbuffers/pyi,
but we don't have any real guarantees about how those versions map to
the runtime version. I think we ought to loosen them to just allow the
user to figure out the version they want. Again, we can revert this, but
I figure we ought to do this update in one-shot.

Author: stefanvanburen

plugins/protocolbuffers/pyi/v30.2/buf.plugin.yaml

@@ -14,5 +14,5 @@ registry:
     requires_python: ">=3.9"
     deps:
       # https://pypi.org/project/protobuf/
-      - "protobuf~=6.30.2"
-      - "types-protobuf~=5.29"
+      - "protobuf>=3.20"
+      - "types-protobuf"

plugins/protocolbuffers/pyi/v31.1/buf.plugin.yaml

@@ -14,5 +14,5 @@ registry:
     requires_python: ">=3.9"
     deps:
       # https://pypi.org/project/protobuf/
-      - "protobuf~=6.31.1"
-      - "types-protobuf~=6.30"
+      - "protobuf>=3.20"
+      - "types-protobuf"

plugins/protocolbuffers/pyi/v32.1/buf.plugin.yaml

@@ -14,6 +14,6 @@ registry:
     requires_python: ">=3.9"
     deps:
       # https://pypi.org/project/protobuf/
-      - "protobuf>=6.32.1"
+      - "protobuf>=3.20"
       # https://pypi.org/project/types-protobuf/
-      - "types-protobuf>=6.32"
+      - "types-protobuf"

plugins/protocolbuffers/python/v30.2/buf.plugin.yaml

@@ -16,4 +16,4 @@ registry:
     requires_python: ">=3.9"
     deps:
       # https://pypi.org/project/protobuf/
-      - "protobuf~=6.30.2"
+      - "protobuf>=3.20"

plugins/protocolbuffers/python/v31.1/buf.plugin.yaml

@@ -16,4 +16,4 @@ registry:
     requires_python: ">=3.9"
     deps:
       # https://pypi.org/project/protobuf/
-      - "protobuf~=6.31.1"
+      - "protobuf>=3.20"

plugins/protocolbuffers/python/v32.1/buf.plugin.yaml

@@ -16,4 +16,4 @@ registry:
     requires_python: ">=3.9"
     deps:
       # https://pypi.org/project/protobuf/
-      - "protobuf>=6.32.1"
+      - "protobuf>=3.20"