Assert on version number format, not exact number

It’s annoying to have to bump the tests every time the version number
changes. This commit changes the assertion to just check that the
version number is in the correct format, not that it’s an exact match.

The regex is adapted from
https://semver.org/#is-there-a-suggested-regular-expression-regex-to-check-a-semver-string
which looks like:
```regex
^(0|[1-9]\d*)\.(0|[1-9]\d*)\.(0|[1-9]\d*)(?:-((?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*))*))?(?:\+([0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*))?$
```

I’ve cut it down to only look at MAJOR.MINOR.PATCH, since we don’t do
alpha/beta/pre-release versions of this library.

Other options we chose not to go with:
- delete this test entirely
- `assert […].startswith('NOTIFY-API-PYTHON-CLIENT')`

We used to have the latter and it caused a bug where the version number
in the user agent was set to `None`. We think this is why the version
number was added to the test in https://github.com/alphagov/notifications-python-client/pull/67