sslutil: eliminate `_canloaddefaultcerts` by constant-folding code using it

tests: remove "defaultcacerts" check `sslutil._canloaddefaultcerts` is always true (and will be removed).

sslutil: eliminate `modernssl` by constant-folding code using it

hgweb: avoid using `sslutil.modernssl` `sslutil.modernssl` is going to be removed. Since the point of using this attribute was to check the importability of the `sslutil`, a different attribute can be used. `sslutil.wrapserversocket` is used because it’s anyway used a few lines below.

sslutil: remove comments referring to removed SSLContext emulation class

sslutil: remove code checking for presence of ssl.SSLContext Now that we require the presence of ssl.SSLContext in setup.py, we can remove this code.

setup: require a Python version with modern SSL features This increases the minimum security baseline of Mercurial and enables us to remove compatibility code for supporting older, less secure Python versions.

sslutil: set `_canloaddefaultcerts` to `True` if `ssl.SSLContext` is present The `load_default_certs()` method was already present when `ssl.SSLContext` was backported to Python 2.7 (https://hg.python.org/cpython/rev/221a1f9155e2).

filemerge: add __bytes__ for absentfilectx This will at _least_ aid some upcoming debugging. Differential Revision: https://phab.mercurial-scm.org/D8592

mergestate: move staticmethod _filectxorabsent to module level I suspect this was a static method just because it made merge.py feel less messy, but now we have a mergestate package so we can do better. Differential Revision: https://phab.mercurial-scm.org/D8591