Annotations in `tests`, part 1

[DMRobertson]

Draft for now while CI runs.

I started by adding annotations to tests.utils like I did the rest of the project, using the same strict config as before. Then I took a few modules and ran mypy on them, and dealt with any unexpected complaints reported by mypy. Then I suppressed the uninteresting errors in mypy config.

Its complaints so far:

• test functions aren't annotated. For simple functions this is just disallow_untyped_defs being unhappy that we haven't marked test function as -> None. There are some parameterised tests though, and I found it useful to add in annotations here to ensure the test body was doing something sensible.
  • FWIW I wouldn't be against adding the -> None
• decorators from pytest and hypothesis return functions whose type involves Any. Not interesting; ignored in config.
• Mypy spotted that crypto_scalarmult_BYTES and SCALARBYTES were untyped, see 10434eb. Easy to fix.
• Also in 10434eb, we were calling crypto_box with a string nonce. I'm a bit surprised this worked, to be honest. I guess the fact that the nonce was ascii-encodable (i.e. just nulls) might have helped?

I didn't feel like annotating check_type_error three times, so I pulled it out to tests.utils. It's a bit naughty to do so in this PR (could spin it out?) and I felt a bit icky making utils pull in pytest.

[reaperhulk]

Huge thanks for working through this big project with me. I think this is the last thing I would consider a blocker for the next pynacl release, so now it's just some housekeeping and getting a release out the door in the next few days/weeks.

[DMRobertson]

Huge thanks for working through this big project with me.

My pleasure---thanks for reviewing!