check-py3-compat: drop support for Python 2
We no longer support Python 2 so we can drop support for linting code
for Python 2 support.
This gets rid of the check for `from __future__`, enabling us to delete
those imports.
Differential Revision: https://phab.mercurial-scm.org/D12251
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif