Replace usage of google.api.core with google.api_core (#4221)

* Remove api.core packages from google.cloud.core, make google.cloud.core depend on api_core.

* s/google.api.core/google.api_core/g and nox updates

* Fixing core tests, addressing review feedback

* Fix bigquery
diff --git a/google/api_core/future/polling.py b/google/api_core/future/polling.py
index eea9624..b4cb5ff 100644
--- a/google/api_core/future/polling.py
+++ b/google/api_core/future/polling.py
@@ -100,8 +100,8 @@
             google.protobuf.Message: The Operation's result.
 
         Raises:
-            google.gax.GaxError: If the operation errors or if the timeout is
-                reached before the operation completes.
+            google.api_core.GoogleAPICallError: If the operation errors or if
+                the timeout is reached before the operation completes.
         """
         self._blocking_poll(timeout=timeout)
 
@@ -120,7 +120,8 @@
                 If None, wait indefinitely.
 
         Returns:
-            Optional[google.gax.GaxError]: The operation's error.
+            Optional[google.api_core.GoogleAPICallError]: The operation's
+                error.
         """
         self._blocking_poll()
         return self._exception
diff --git a/nox.py b/nox.py
index bb10c6c..b614e46 100644
--- a/nox.py
+++ b/nox.py
@@ -42,7 +42,6 @@
     session.run(
         'py.test',
         '--quiet',
-        '--cov=google.cloud',
         '--cov=google.api_core',
         '--cov=tests.unit',
         '--cov-append',