`refresh_certificate` used to build a new `ServerContextFactory` on
every SIGHUP and then replace the factory on each listening port with a
fresh `TLSMemoryBIOFactory` wrapping the new one, reaching into the
port's `factory` attribute to do so.
Twisted asks the context factory for a context on every new connection,
so rebuilding the `SSL.Context` inside the existing factory is enough
for new connections to pick up the new certificate. `start()` now builds
the factory once, and `refresh_certificate` is only the SIGHUP handler.
This is spawning of #20452: Twisted 26.10 annotates `tcp.Port.factory`
as `Factory | None`, and mypy does not accept a `TLSMemoryBIOFactory`
there because its protocol type parameter does not match, so the old
assignment no longer type-checks. The change removes the assignment
rather than suppressing the error.
Verified with trial against Twisted 23.8.0 (our minimum declared dep),
26.4.0 (the current version in the lockfile) and 26.10.0 (the next
version), and with mypy against 26.4.0 and 26.10.0.
