#24082 [python] honor proxy environment settings

The default urllib3 client does not read proxy environment variables,
so generated clients require users to copy them into
Configuration.proxy. 97e079fd added no_proxy handling, but
01ed5975 replaced the Python templates without carrying it forward.

Read scheme-specific proxies and no_proxy from the standard library's
environment proxy configuration. Explicit empty values remain opt-outs,
and bypass matching covers domains, ports, IPv4 CIDRs, and IPv6 CIDRs
without adding requests as a generated dependency.

This is limited to the synchronous urllib3 client. The async backends
have separate proxy implementations.

Fixes #6786.
Partially addresses #20226.