tests: check ETag format in test-hgweb-commands

hgweb: emit a valid, weak ETag Previously, ETag headers from hgweb weren't correctly formed, because rfc2616 (section 14, header definitions) requires double quotes around the content of the header. str(web.mtime) didn't do that. Additionally, strong ETags signify that the resource representations are byte-for-byte identical. That is, they can be reconstructed from byte ranges if client so wishes. Considering ETags for all hgweb pages is just mtime of 00changelog.i and doesn't consider of e.g. .hg/hgrc with description, contact and other fields, it's clearly shouldn't be strong. The W/ prefix marks it as weak, which still allows caching the whole served file/page, but doesn't allow byte-range requests.

policy: add cffi policy for PyPy This adds cffi policy in the case where we don't want to use C modules, but instead we're happy to rely on cffi (bundled with pypy)

sslutil: handle default CA certificate loading on Windows See the inline comment for what's going on here. There is magic built into the "ssl" module that ships with modern CPython that knows how to load the system CA certificates on Windows. Since we're not shipping a CA bundle with Mercurial, if we're running on legacy CPython there's nothing we can do to load CAs on Windows, so it makes sense to print a warning. I don't anticipate many people will see this warning because the official (presumed popular) Mercurial distributions on Windows bundle Python and should be distributing a modern Python capable of loading system CA certs.

sslutil: expand _defaultcacerts docstring to note calling assumptions We should document this so future message additions don't seem out of place.

sslutil: document the Apple OpenSSL cert trick This is sort of documented in _plainapplypython()'s docstring. But it helps to be explicit in security code.

sslutil: use certificates provided by certifi if available The "certifi" Python package provides a distribution of the Mozilla trusted CA certificates as a Python package. If it is present, we assume the user intends it to be used and we use it to provide the default CA certificates when certificates are otherwise not configured. It's worth noting that this behavior roughly matches the popular "requests" package, which also attempts to use "certifi" if present.

py3: make files use absolute_import and print_function This patch includes addition of absolute_import and print_function to the files where they are missing. The modern importing conventions are also followed.

sslutil: don't attempt to find default CA certs file when told not to Before, devel.disableloaddefaultcerts only impacted the loading of default certs via SSLContext. After this patch, the config option also prevents sslutil._defaultcacerts() from being called. This config option is meant to be used by tests to force no CA certs to be loaded. Future patches will enable _defaultcacerts() to have success more often. Without this change we can't reliably test the failure to load CA certs. (This patch also likely fixes test failures on some OS X configurations.)

sslutil: pass ui to _defaultcacerts We'll use this shortly.