pytest-httpbin<1.0 ships with a server certificate with a commonName but
no subjectAltName. urllib3 2.0 will stop supporting those in the future,
so we want to upgrade pytest-httpbin.
Unfortunately, `test_https_warnings` was relying on this broken
certificate. With this change, we use `trustme` to create a broken
certificate specifically for this test, so that we can upgrade
pytest-httpbin and make sure that other tests relying on httpbin TLS
support will continue to work with urllib3 2.0.
Nate Prewitt