pythonPackages.protobuf: apply patch for python 3.7 only when buildin…

…g with it

Change introduced in NixOS#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.

pkgs/development/python-modules/protobuf/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchpatch, python, buildPythonPackage
+{ stdenv, fetchpatch, python, buildPythonPackage, isPy37
 , protobuf, google_apputils, pyext, libcxx
 , disabled, doCheck ? true }:
 
@@ -16,14 +16,14 @@ buildPythonPackage rec {
   propagatedBuildInputs = [ protobuf google_apputils ];
   buildInputs = [ google_apputils pyext ];
 
-  patches = [
+  patches = optional isPy37
     # Python 3.7 compatibility (remove when protobuf 3.7 is released)
     (fetchpatch {
       url = "https://github.com/protocolbuffers/protobuf/commit/0a59054c30e4f0ba10f10acfc1d7f3814c63e1a7.patch";
       sha256 = "09hw22y3423v8bbmc9xm07znwdxfbya6rp78d4zqw6fisdvjkqf1";
       stripLen = 1;
     })
-  ];
+  ;
 
   prePatch = ''
     while [ ! -d python ]; do