Make Datastore doctests use a namespace.

[lukesneeringer]

Fixes #3232.

Running nox -s doctests passes locally (I expect CI will not check it until post-merge):

$ nox -s doctests
nox > Running session doctests
nox > The virtualenv name was hashed to avoid being too long.
nox > virtualenv /usr/local/google/home/lukesneeringer/Code/Google/google-cloud-python/datastore/.nox/765eb60c -p python3.6
nox > chdir /usr/local/google/home/lukesneeringer/Code/Google/google-cloud-python/datastore
nox > pip install --upgrade mock pytest sphinx ../core/
nox > pip install --upgrade ../test_utils/
nox > pip install --upgrade .
nox > py.test --quiet tests/doctests.py
.
1 passed in 12.24 seconds
nox > Session doctests successful. :)

[lukesneeringer]

P.S. It is really hard to edit anything with three-space indenting.

[dhermes]

Cursory LGTM.

Note there is still an open issue (#3481) because these doctests are not performing cleanup as expected. When I last dug into it, the issue was the difference between the behavior of testcleanup (in Sphinx) between Python 2 and 3.