hghave: add test for Python 2.7+

Setting ciphers in the ssl module requires Python 2.7. Surprisingly,
we didn't have a test for running on Python 2.7.

tests/hghave.py

@@ -523,6 +523,10 b' def has_absimport():'
     from mercurial import util
     return util.safehasattr(__future__, "absolute_import")
 
+@check("py27+", "running with Python 2.7+")
+def has_python27ornewer():
+    return sys.version_info[0:2] >= (2, 7)
+
 @check("py3k", "running with Python 3.x")
 def has_py3k():
     return 3 == sys.version_info[0]