Get doctests in shape to be tested by tox

Make tox test all the doctests in the Sphinx docs.

Ours aren’t in shape to do that (yet, might be hard to get there).

See comment by @tseaver for more context

[dhermes]

@daspecster As seen above (I edited after original posting), doctest.run_docstring_examples doesn’t actually fail in the case of an error, so using the unit test runner allows us to see failures as exceptions / nonzero exit codes.

[dhermes]

Please file an issue saying “I’m working on storage snippets”.

---

In storage_snippets.py you would do the following:

SNIPPET_GET_BLOB = """\
>>> bucket = client.get_bucket('my-bucket')
>>> print(bucket.get_blob('/path/to/blob.txt'))
<Blob: my-bucket, /path/to/blob.txt>
>>> print(bucket.get_blob('/does-not-exist.txt'))
None
"""

def get_blob_snippet():
    from google.cloud import storage
    client = storage.Client()
    # Other setup?

    fake_mod = types.ModuleType('fake-mod')
    fake_mod.__doc__ = SNIPPET_GET_BLOB
    fake_globals = {'client': client}
    doctest.run_docstring_examples(fake_mod, fake_globals, verbose=False, name='get_blob')