setup: require that Python has TLS 1.1 or TLS 1.2 This ensures that Mercurial never downgrades the minimum TLS version from TLS 1.1+ to TLS 1.0+ and enables us to remove that compatibility code. It is reasonable to expect that distributions having Python 2.7.9+ or having backported modern features to the ssl module (which we require) have a OpenSSL version supporting TLS 1.1 or TLS 1.2, as this is the main reason why distributions would want to backport these features. TLS 1.1 and TLS 1.2 are often either both enabled or both not enabled. However, both can be disabled independently, at least on current Python / OpenSSL versions. For the record, I contacted the CPython developers to remark that unconditionally defining ssl.PROTOCOL_TLSv1_1 / ssl.PROTOCOL_TLSv1_2 is problematic: https://github.com/python/cpython/commit/6e8cda91d92da72800d891b2fc2073ecbc134d98#r39569316

--- a/relnotes/next	Sun May 31 12:07:17 2020 +0200
+++ b/relnotes/next	Sat May 30 23:42:19 2020 +0200
@@ -7,7 +7,9 @@
 == Backwards Compatibility Changes ==
 
 * Mercurial now requires at least Python 2.7.9 or a Python version that
-  backported modern SSL/TLS features (as defined in PEP 466).
+  backported modern SSL/TLS features (as defined in PEP 466), and that Python
+  was compiled against a OpenSSL version supporting TLS 1.1 or TLS 1.2
+  (likely this requires the OpenSSL version to be at least 1.0.1).
 
 
 == Internal API Changes ==