python3Packages.sphinxcontrib-openapi: disable py2

```
builder for '/nix/store/jz41wvwvkc6q55972ysdajm6k3vnv6r8-python2.7-sphinxcontrib-openapi-0.7.0.drv' failed with exit code 1; last 10 log lines:
  configuring
  no configure script, doing nothing
  building
  Executing setuptoolsBuildPhase
  Traceback (most recent call last):
    File "nix_run_setup", line 8, in <module>
      exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\\r\\n', '\\n'), __file__, 'exec'))
    File "setup.py", line 10, in <module>
      with open(os.path.join(here, "README.rst"), "r", encoding="utf-8") as f:
  TypeError: 'encoding' is an invalid keyword argument for this function
```

pkgs/development/python-modules/sphinxcontrib-openapi/default.nix

@@ -1,6 +1,7 @@
 { stdenv
 , buildPythonPackage
 , fetchPypi
+, isPy27
 , setuptools_scm
 , m2r
 , pyyaml
@@ -11,6 +12,7 @@
 buildPythonPackage rec {
   pname = "sphinxcontrib-openapi";
   version = "0.7.0";
+  disabled = isPy27;
 
   src = fetchPypi {
     inherit pname version;