hgweb: put response headers back into str for Python 3 This fixes a lot of hanging tests on Python 3, because "Content-Length" was getting sent as "b'Content-Length'" (yes, really) and then clients would expect a close-is-end body instead of counting off a certain number of bytes. Differential Revision: https://phab.mercurial-scm.org/D3248

tests: load showstack in test-pull-http.t so network hangs are easier to find This also gives us some minimal "it loads" coverage on showstack, which I rather like. showstack doesn't work on Windows per mbarbison, so it's disabled there. I added this in service of debugging a hang introduced on Python 3 by revision a88d68dc3ee8. I'm still not sure what the problem there is, but this at least gives us a little bit of a chance to figure out what's going on. Differential Revision: https://phab.mercurial-scm.org/D3247

keepalive: rewrite readinto() to not use read() It turns out http.client on Python 3 sometimes uses readinto() in the implementation of read(). Unfortunately, Python 2 doesn't have a readinto() in httplib's client, so we have to support both codepaths. Subclassing is bad, folks. Differential Revision: https://phab.mercurial-scm.org/D3246