add some basic testing

Closes #2

See merge request !2

Add basic GitLab CI configuration based on the current webapp
boilerplate. We test "naked" pytest to ensure that VS Code based testing
workflows continue to work along with more traditional tox-based tests.

We test a Python/Django matrix based on the list of compatible versions
from the Django website.

Add check for basic functionality which just ensures that the expected
methods in the snapshot client object were called.

Add a pytest/tox based test harness based on the webapp boilerplate.
This tox test harness allows for multiple Django versions to be tested
and ships a simple Django project settings for use in tests.

Add dependencies to a "dev" group which are required to be able to run
tests.

As a library, we want to support as wide a selection of Django versions
as possible. Widen the required library versions.

Add standard configuration for Python utilities from boilerplate.

It isn't terribly Pythonic to raise a bare exception. Instead we prefer
to raise an exception based on one of the standard errors. Create our
own "SnapshotError" based on "RuntimeError" and use that instead.

Initial development

Closes #1

See merge request !1