protobuf: apply patch for python 3.7 only when building with it

[samueldr]

Change introduced in #50672.

The patch does not apply on older protobuf like protobuf3_1

$ nix-build -E 'with import ./. { }; python3Packages.protobuf.override { protobuf = protobuf3_1; }'
unpacking sources
unpacking source archive /nix/store/1zdyl0cxaa8ha2v1zp75zzdjd6j99d0m-source
source root is source
setting SOURCE_DATE_EPOCH to timestamp 315619200 of file source/util/python/BUILD
patching sources
applying patch /nix/store/yagx7hvylnnjq7lxbcia0y5lq1r736w3-0a59054c30e4f0ba10f10acfc1d7f3814c63e1a7.patch
patching file google/protobuf/pyext/descriptor.cc
Hunk #1 succeeded at 55 (offset -1 lines).
patching file google/protobuf/pyext/descriptor_containers.cc
patching file google/protobuf/pyext/descriptor_pool.cc
Hunk #1 succeeded at 47 (offset -1 lines).
patching file google/protobuf/pyext/extension_dict.cc
Hunk #1 FAILED at 53.
1 out of 1 hunk FAILED -- saving rejects to file google/protobuf/pyext/extension_dict.cc.rej
patching file google/protobuf/pyext/message.cc
Hunk #1 succeeded at 82 (offset 3 lines).
Hunk #2 succeeded at 1425 (offset -104 lines).

Since the patch isn't necessary on python versions older than 3.7, let's
only apply it for version 3.7. This means that most things using older
protobuf implementation will now be able to build when using an older
pythonPackage set (as is most probably the case anyway).

This still leaves protobuf 3.1 using packages hanging, but the errors
will be localized to those that would be breaking anyway with the
upgrade to 3.7 as default python.

This can be tested using:

nix-build -E 'with import ./. { }; python36Packages.protobuf.override { protobuf = protobuf3_1; }'

Things done

• ✔️ Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • ✔️ NixOS
  • ⬜ macOS
  • ⬜ other Linux distributions
• ⬜ Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• ⬜ Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• ⬜ Tested execution of all binary files (usually in ./result/bin/)
• ⬜ Determined the impact on package closure size (by running nix path-info -S before and after)
• ✔️ Assured whether relevant documentation is up to date
• ✔️ Fits CONTRIBUTING.md.

---

cc @lopsided98 @worldofpeace

[samueldr]

See #51809 for a package this fixes. Since the patch won't apply with python 3.6, changing to python 3.6 is not enough to fix caffe2.

[worldofpeace]

Wouldn't pythonAtLeast make more sense here?

[FRidh]

isPy37

[samueldr]

Thanks! the things you can learn about the python ecosystem ;).

[samueldr]

Updated with isPy37.

[dotlambda]

I'll merge #51809 once nix-review is done, hence closing this one.