REQUIRES SR#1120297

- Update to 0.25.0:
- Drop support for Python 3.7.
- Support HTTPS proxies.
- Change the type of Extensions from Mapping[Str, Any] to
MutableMapping[Str, Any].
- Add socket_options argument to httpx.HTTPTransport and
httpx.AsyncHTTPTransport classes.
- The Response.raise_for_status() method now returns
the response instance. For example: data =
httpx.get('...').raise_for_status().json().
- Return 500 error response instead of exceptions when
raise_app_exceptions=False is set on ASGITransport.
- Ensure all WSGITransport environs have a SERVER_PROTOCOL.
- Always encode forward slashes as %2F in query parameters
- Use Mozilla documentation instead of httpstatuses.com for
HTTP error reference
- Requires higher version of httpcore.