Close transports

[gbenson]

This fix implements RetryTransport.close() and .aclose() so the inner transports are freed when httpx.Client.close() or httpx.Client.aclose() is called. This avoids resource warnings about leaked SSL sockets when Python exits, e.g.:

ResourceWarning: unclosed <ssl.SSLSocket fd=11, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('192.168.1.6', 40038), raddr=('18.245.162.93', 443)>

Decisions:

• I wasn't sure whether to close transport passed as arguments to RetryTransport.__init__, so I implemented the same behavior as httpx.Client.close(), which is to close all transports irrespective of where they came from.
• RetryTransport.__init__ creates both sync and async transports if you don't pass one in, but RetryTransport.close() and .aclose() only close their own transport. I don't think this would cause problems since the one you close is likely the one you use, so the transport you don't close probably didn't ever open any sockets.

[will-ockmore]

Thanks for this @gbenson , your decisions are reasonable ones. I'll include in the next release.