Add support for SSLKEYLOGFILE for Python 3.8b4+ #299

Python 3.8b4 (with OpenSSL 1.1.1) added support for the SSLKEYLOGFILE environment variable which will be very useful for using Wireshark to do analysis of HTTP/2 requests for example.

This should be a small change within SSLConfig._create_default_ssl_context() see this commit for the gist of how to implement.

Would also require that we document within the Environment Variables section that we support this environment variable and under what circumstances.