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
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