Typing Improvement - Client mounts

[tomtom103]

The httpx.Client defines the self._mounts as Dict[URLPattern, typing.Optional[BaseTransport]], but the constructor argument is typed as: Optional[Mapping[str, BaseTransport]].

Following the logic described in https://www.python-httpx.org/advanced/#no-proxy-support, when manually providing mounts to the client (for example, disabling certain features for a specific domain), we should be able to provide a None transport to ensure the transport used will be the one provided by default to the client instead of matching with one of the mounts.

Take the following example:

import httpx

mounts = {
    "http://": httpx.HTTPTransport(proxy=httpx.Proxy(url="http://my-proxy:8030")),
    "https://": httpx.HTTPTransport(proxy=httpx.Proxy(url="http://my-proxy:8030")),
    "all://*.my-domain.com": None # Disable all proxying to specific domain and use specified transport instead
}

with httpx.Client(
    transport=httpx.HTTPTransport(verify=False),
    mounts=mounts
) as client:
    # Should not use a proxy
    # Defaults to the provided transprot
    response = client.get("https://foo.my-domain.com/get")

    # Should use a proxy
    response = client.get("https://httpbin.org/get")

Type-checker fails with:

Argument "mounts" to "Client" has incompatible type "Dict[str, Optional[HTTPTransport]]"; expected "Optional[Mapping[str, BaseTransport]]"

But seems to be a valid use-case here according to the logic defined in the Client._transport_for_url. This is also a valid use-case at runtime, I'm wondering if this is something the types should also support?

[karpetrosyan]

I think it has been resolved in #2879