feat: add data source for Cloud Spanner

[tswast]

Closes #60

[tswast]

I tried to reproduce this issue:

import datetime
import pandas

print(pandas.__version__)

df = pandas.DataFrame({
    "int_col": [1],
    "datetime_col": pandas.Series(
        ["2021-03-12T12:50:25"], dtype="datetime64[ns]"
    ),
})

df["datetime_col"] = df["datetime_col"].dt.tz_localize(datetime.timezone.utc)

for i, v in df.dtypes.iteritems():
    print(f"{i}: {v}")

It was successful in my environment, so I'm not sure what's going on in Python 3.9 session.

[tswast]

Removed because the dtype translation was messing up NULL handling.

[dhercher]

This test might be less error prone using fake fs, plus you wont need to clean up the connection file at the end?

[tswast]

I tried, and am getting an error when the client authenticates. First, it couldn't find my key file, so I used an updated fixture to add it as a real path.

@pytest.fixture
def fs_with_creds(fs):
    if "GOOGLE_APPLICATION_CREDENTIALS" in os.environ:
        fs.add_real_file(os.environ["GOOGLE_APPLICATION_CREDENTIALS"])

    yield fs

But even with this, I get an error:

ERROR    grpc._plugin_wrapping:_plugin_wrapping.py:82 AuthMetadataPluginCallback "<google.auth.transport.grpc.AuthMetadataPlugin object at 0x7fb3b8b9ae10>" raised exception!
Traceback (most recent call last):
  File "/Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/grpc/_plugin_wrapping.py", line 78, in __call__
    context, _AuthMetadataPluginCallback(callback_state, callback))
  File "/Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/google/auth/transport/grpc.py", line 86, in __call__
    callback(self._get_authorization_headers(context), None)
  File "/Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/google/auth/transport/grpc.py", line 73, in _get_authorization_headers
    self._request, context.method_name, context.service_url, headers
  File "/Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/google/auth/credentials.py", line 133, in before_request
    self.refresh(request)
  File "/Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/google/oauth2/service_account.py", line 361, in refresh
    access_token, expiry, _ = _client.jwt_grant(request, self._token_uri, assertion)
  File "/Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/google/oauth2/_client.py", line 153, in jwt_grant
    response_data = _token_endpoint_request(request, token_uri, body)
  File "/Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/google/oauth2/_client.py", line 105, in _token_endpoint_request
    response = request(method="POST", url=token_uri, headers=headers, body=body)
  File "/Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/google/auth/transport/requests.py", line 183, in __call__
    method, url, data=body, headers=headers, timeout=timeout, **kwargs
  File "/Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/requests/sessions.py", line 542, in request
    resp = self.send(prep, **send_kwargs)
  File "/Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/requests/sessions.py", line 655, in send
    r = adapter.send(request, **kwargs)
  File "/Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/requests/adapters.py", line 416, in send
    self.cert_verify(conn, request.url, verify, cert)
  File "/Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/requests/adapters.py", line 228, in cert_verify
    "invalid path: {}".format(cert_loc))
OSError: Could not find a suitable TLS CA certificate bundle, invalid path: /Users/swast/miniconda3/envs/pso-data-validator-ibis-1.4/lib/python3.7/site-packages/certifi/cacert.pem

I believe it's having trouble reading the root certificates to validate a secure connection.

[tswast]

I could try adding some more "real" files / directories, but I think it's becoming more error-prone, not less :-(

[dhercher]

This looks good to go beyond the one nit

[dhercher]