Phase 1

• Add an HTTP/2-capable handler to the test suite #3038
• Migrate HTTPDummyServerTestCase to Hypercorn #3196
• Migrate run_server_in_thread and run_server_and_proxy_in_thread to Hypercorn #3197
• Create a mechanism to test different HTTP protocols (HTTP/1.1 and HTTP/2) with the same test case #3194
• Create a new submodule (urllib3.http2) which will house all the logic for implementing HTTP/2 which is only available if the h2 module is installed.
• Create additional types/parameters/properties to the existing BaseHTTPConnection and BaseHTTPResponse, etc that will be needed for HTTP/2 support (http_version, alpn_protocols)
• Decide architecture for HTTP/2 multiplexing (HTTPSConnection or HTTPSConnectionPool source of multiplexing?)

Phase 2

• Migrate socket-level tests to support HTTP/2
• Create a new HTTPS connection class which supports HTTP/2.
  • Rudimentary HTTP/2 connection and response #3284
  • Keep the connection open and reuse it, update the window while reading response.
  • Add support for streaming HTTP/2 responses #3300
  • Add support for more than one stream per socket connection. Buffering read data for other connections.
• Ensure HTTP/2 isn't offered for tunnel CONNECT requests #3298
• Add HTTP/2 support for forwarding proxies #3299
• Ensure this connection class can pass all existing test suites for HTTP connections
• Map HTTP/2 connection and stream error codes to existing urllib3 exceptions
• Implement multiplexing architecture decided in phase 1
• Experiment with backgroundable stream management for window sizing, ping requests, etc.

Phase 3

• Compatibility testing with services
• Performance testing
• Closed beta with biggest users and projects
• Create opt-in mechanism for open beta (alpn_protocols?)
• Final release with opt-in mechanism enabled by default.