circle: fix pip install grpcio breakage

example failure: https://app.circleci.com/pipelines/github/snarfed/oauth-dropins/90/workflows/2fb906b4-78f4-45d7-ad4d-46ed9d28b878/jobs/516

full error log:

```
Running setup.py install for grpcio ... error
  Complete output from command /root/project/local3/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-2x6fpemn/grpcio/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-record-tmyhem3j/install-record.txt --single-version-externally-managed --compile --install-headers /root/project/local3/include/site/python3.7/grpcio:
  Cython-generated files are missing...
  We could not find Cython. Setup may take 10-20 minutes.
  WARNING: The wheel package is not available.
...
Traceback (most recent call last):
  File "/tmp/pip-install-mlr1xc5w/grpcio/src/python/grpcio/commands.py", line 264, in build_extensions
    build_ext.build_ext.build_extensions(self)
  File "/usr/lib/python3.7/distutils/command/build_ext.py", line 449, in build_extensions
    self._build_extensions_serial()
  File "/usr/lib/python3.7/distutils/command/build_ext.py", line 474, in _build_extensions_serial
    self.build_extension(ext)
  File "/root/project/local3/lib/python3.7/site-packages/setuptools/command/build_ext.py", line 200, in build_extension
    if ext._needs_stub:
AttributeError: 'Extension' object has no attribute '_needs_stub'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/tmp/pip-install-mlr1xc5w/grpcio/setup.py", line 465, in <module>
    cmdclass=COMMAND_CLASS,
  File "/root/project/local3/lib/python3.7/site-packages/setuptools/__init__.py", line 145, in setup
    return distutils.core.setup(**attrs)
  File "/usr/lib/python3.7/distutils/core.py", line 148, in setup
    dist.run_commands()
  File "/usr/lib/python3.7/distutils/dist.py", line 966, in run_commands
    self.run_command(cmd)
  File "/usr/lib/python3.7/distutils/dist.py", line 985, in run_command
    cmd_obj.run()
  File "/root/project/local3/lib/python3.7/site-packages/setuptools/command/install.py", line 61, in run
    return orig.install.run(self)
  File "/usr/lib/python3.7/distutils/command/install.py", line 589, in run
    self.run_command('build')
  File "/usr/lib/python3.7/distutils/cmd.py", line 313, in run_command
    self.distribution.run_command(command)
  File "/usr/lib/python3.7/distutils/dist.py", line 985, in run_command
    cmd_obj.run()
  File "/usr/lib/python3.7/distutils/command/build.py", line 135, in run
    self.run_command(cmd_name)
  File "/usr/lib/python3.7/distutils/cmd.py", line 313, in run_command
    self.distribution.run_command(command)
  File "/usr/lib/python3.7/distutils/dist.py", line 985, in run_command
    cmd_obj.run()
  File "/root/project/local3/lib/python3.7/site-packages/setuptools/command/build_ext.py", line 78, in run
    _build_ext.run(self)
  File "/usr/lib/python3.7/distutils/command/build_ext.py", line 340, in run
    self.build_extensions()
  File "/tmp/pip-install-mlr1xc5w/grpcio/src/python/grpcio/commands.py", line 267, in build_extensions
    support.diagnose_build_ext_error(self, error, formatted_exception)
  File "/tmp/pip-install-mlr1xc5w/grpcio/src/python/grpcio/support.py", line 118, in diagnose_build_ext_error
    diagnostic(build_ext, error)
  File "/tmp/pip-install-mlr1xc5w/grpcio/src/python/grpcio/support.py", line 98, in diagnose_attribute_error
    "We expect a missing `_needs_stub` attribute from older versions of "
commands.CommandError: We expect a missing `_needs_stub` attribute from older versions of setuptools. Consider upgrading setuptools.

----------------------------------------
Command "/root/project/local3/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-mlr1xc5w/grpcio/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-record-grvcioh0/install-record.txt --single-version-externally-managed --compile --install-headers /root/project/local3/include/site/python3.7/grpcio" failed with error code 1 in /tmp/pip-install-mlr1xc5w/grpcio/
```

.circleci/config.yml

@@ -27,6 +27,7 @@ jobs:
             apt-get install -y python3.7 python3.7-venv python3.7-dev
             python3.7 -m venv local3
             . local3/bin/activate
+            pip install cython  # avoids grpcio install bug (search _needs_stub)
             pip install -U -r requirements.txt
             pip install mox3
             pip install coverage coveralls