Update `requirements.txt` and tests to get them passing without being skipped again

[grahamalama]

• update requirements.txt by running pip-compile --generate-hashes
  • dependabot indeed had a PR up for this, we just didn't merge it in a timely manner
• mark async tests via pytest_asyncio.fixture and setting asyncio_mode = "auto"
• fixed errors caused by mock_cert (see commit message of https://github.com/mozilla-services/python-autograph-utils/commit/f6b88685f72098cb2d114a57888d10018fd17793 for a detailed explanation)

[grahamalama]

https://github.com/mozilla-services/python-autograph-utils/pull/42/commits/86f1f842850a0fe37318ada90e20c909257fa6bf 😮‍💨 only the first circle of dependency hell, but I don't know why I had to put this back manually.

That likely won't be sustainable as it might be overwritten the next time we do pip-compile --generate-hashes.