.arcconfig

hgweb: inline caching() and port to modern mechanisms

We only had one consumer of this simple function. While it could be
a generic function, let's not over abstract the code.

As part of inlining, we port it off wsgirequest, fix some Python 3
issues, and set a response header on our new response object so it
is ready once we start using it to send responses.

Differential Revision: https://phab.mercurial-scm.org/D2785

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