deps: require google-cloud-bigquery-storage to avoid performance warn…

…ing (#414)

The wrapped DB-API uses google-cloud-bigquery-storage to download query
results by default. It raises a warning and falls back to the REST API
if google-cloud-bigquery-storage is not installed.

The google-cloud-bigquery version 3.x library is tested in #401 and it does not include any breaking changes that should affect the SQLAlchemy connector.

deps: allow google-cloud-bigquery version 3.x

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-bigquery-sqlalchemy/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

setup.py

@@ -84,7 +84,9 @@ def readme():
         # Until this issue is closed
         # https://github.com/googleapis/google-cloud-python/issues/10566
         "google-auth>=1.25.0,<3.0.0dev",  # Work around pip wack.
-        "google-cloud-bigquery>=2.25.2,<3.0.0dev",
+        "google-cloud-bigquery>=2.25.2,<4.0.0dev",
+        "google-cloud-bigquery-storage>=2.0.0,<3.0.0dev",
+        "pyarrow>=3.0.0,<7.0dev",
         # Temporarily set maximimum sqlalchemy to a known-working version while
         # we debug failing compliance tests. See:
         # https://github.com/googleapis/python-bigquery-sqlalchemy/issues/386

testing/constraints-3.6.txt

@@ -7,4 +7,6 @@
 sqlalchemy==1.2.0
 google-auth==1.25.0
 google-cloud-bigquery==2.25.2
+google-cloud-bigquery-storage==2.0.0
 google-api-core==1.31.5
+pyarrow==3.0.0