typing: Make use of ParamSpec

[stephenfin]

So we can catch type issues like the below:

def foo(a: str, b: int | None = None):
    ...

class FooTest(testtools.TestCase):
    def test_foo(self):
        self.assertRaises(Exception, foo, 123, b='abc')

We don't currently have type checking enabled for the tests directory, but adding a test like the above proves this works.

[stephenfin]

We don't currently have type checking enabled for the tests directory, but adding a test like the above proves this works.

I'm started working on a fix for this in #579 but, as noted there, it's going to take a while 😅