fix: fix socket leak in impersonated_credentials

[stewartmiles]

impersonated_credentials.Credentials.sign_bytes() created
a session that wasn't closed leaking a socket. This fixes
the issue by always closing the requests session after
a signing request is complete.

Fixes #1122

[stewartmiles]

The failing presubmit looks like nothing to do with my change https://source.cloud.google.com/results/invocations/29f9a2fd-8d3a-4251-88f0-4598f6910f6b/targets/github%2Fgoogle-auth-library-python%2Fsystem_tests/tests

verify_refresh = <function _verify_refresh at 0x7fb0f2b48bd0>

    def test_application_default_credentials(verify_refresh):
        credentials, project_id = google.auth.default()

        if EXPECT_PROJECT_ID is not None:
            assert project_id is not None

>       verify_refresh(credentials)

system_tests_sync/test_default.py:28:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
system_tests_sync/conftest.py:122: in _verify_refresh
    credentials.refresh(http_request)
.nox/default_explicit_authorized_user-2-7/lib/python2.7/site-packages/google/oauth2/credentials.py:310: in refresh
    enable_reauth_refresh=self._enable_reauth_refresh,
.nox/default_explicit_authorized_user-2-7/lib/python2.7/site-packages/google/oauth2/reauth.py:347: in refresh_grant
    _client._handle_error_response(response_data)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

response_data = {'error': 'invalid_grant', 'error_description': 'Token has been expired or revoked.'}

    def _handle_error_response(response_data):
        """Translates an error response into an exception.

        Args:
            response_data (Mapping | str): The decoded response data.

        Raises:
            google.auth.exceptions.RefreshError: The errors contained in response_data.
        """
        if isinstance(response_data, six.string_types):
            raise exceptions.RefreshError(response_data)
        try:
            error_details = "{}: {}".format(
                response_data["error"], response_data.get("error_description")
            )
        # If no details could be extracted, use the response data.
        except (KeyError, ValueError):
            error_details = json.dumps(response_data)

>       raise exceptions.RefreshError(error_details, response_data)
E       RefreshError: ('invalid_grant: Token has been expired or revoked.', {u'error_description': u'Token has been expired or revoked.', u'error': u'invalid_grant'})

.nox/default_explicit_authorized_user-2-7/lib/python2.7/site-packages/google/oauth2/_client.py:62: RefreshError

[clundin25]

The failing presubmit looks like nothing to do with my change https://source.cloud.google.com/results/invocations/29f9a2fd-8d3a-4251-88f0-4598f6910f6b/targets/github%2Fgoogle-auth-library-python%2Fsystem_tests/tests

I've pushed a new test token to your branch. The tests will pass now.

[stewartmiles]

@clundin25 thanks, looks like everything is green now :) I don't have a merge button so I assume you'll merge this patch when you're ready?

[clundin25]

@stewartmiles Thanks for the contribution!