On Python 3.12+, urllib.request.HTTPSHandler.__init__ builds a default

SSL context when one isn't passed in, loading the system CA store in the

process. CertificateVerificationHTTPSHandler didn't pass one, so every

handler we construct pays for a context that https_open immediately

throws away and replaces with the managed one from the certificate

manager. Handlers are built per-request, so this isn't free.
This also breaks the cert unit tests, which capture every context

created during a request and then assert against the first one. With the

extra context in the list, they inspect the throwaway instead of ours.
The handler now passes a cheap placeholder context to the parent, which

https_open replaces as before.

Ran unit tests on Python 3.10 and 3.12