As proposed in#135056, Add a --cors command line argument to the stdlib http.server module, which will add anAccess-Control-Allow-Origin: * header to all responses.

Invocation:

python -m http.server --cors

As part of this implementation, add aresponse_headers argument toSimpleHTTPRequestHandler andHTTPServer, which allows callers to add addition headers to the response. Ideally it would have been possible to just have made aCorsHttpServer class, but a couple of issues made that difficult:

• Thehttp.server CLI uses more than one HTTP Server class, in order to support TLS/HTTPS. So a single CorsHttpServer child class wouldn't work to support both use cases.
• Much of the work in specifying HTTP behavior is handled by the variousRequestHandler classes. However, theHttpServer classes didn't have an easy way to pass arguments down into the instantiated handlers.

As a result, this PR updates bothHTTPServer andSimpleHTTPRequestHandler to accept aresponse_headers argument, which allows callers to specify an additional set of HTTP headers to pass in the response.

• HTTPServer now overridesfinish_request to pass this new kwarg down to itsRequestHandler.
• SimpleHTTPRequestHandler now accepts aresposnse_headers kwarg, to optionally specify a dictionary of additional headers to send in the response.

Care is taken to not pass theresponse_headers argument to any instance constructors when not provided, to ensure backwards compatibility. I tried to keep the implementation as short and simple as possible.

With the addition of aresponse_headers argument, we allow ourselves to have a future possible custom header http argument, such as:

python -m http.server -H 'custom-header: custom-value'

• Issue:Add a --cors command line option to stdlib http.server #135056

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