sslutil: avoid deprecation warnings from python 3.10's ssl module
Use ssl.PROTOCOL_TLS_{CLIENT,SERVER} and
SSLContext.{min,max}imum_version when supported (3.7+).
And, catch deprecation warnings when the user asks for deprecated TLS
versions (1.0 and 1.1).
Differential Revision: https://phab.mercurial-scm.org/D12488
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.