extensions: gracefully warn when doing min version check with no local version

After doing a `make clean`, I started getting cryptic failures to import
extensions with the `minimumhgversion` attribute on py3:

    *** failed to import extension evolve: '>' not supported between instances of 'int' and 'NoneType'
    *** failed to import extension topic: '>' not supported between instances of 'int' and 'NoneType'

This now handles the `(None, None)` tuple before comparing, and disables the
extension with the same friendly message as in py2.

Differential Revision: https://phab.mercurial-scm.org/D9363

mercurial/extensions.py

@@ -222,7 +222,10 b' def load(ui, name, path, loadingtime=Non'
     # extensions short circuit when loaded with a known incompatible version
     # of Mercurial.
     minver = getattr(mod, 'minimumhgversion', None)
-    if minver and util.versiontuple(minver, 2) > util.versiontuple(n=2):
+    if minver:
+        curver = util.versiontuple(n=2)
+
+        if None in curver or util.versiontuple(minver, 2) > curver:
         msg = _(
             b'(third party extension %s requires version %s or newer '
             b'of Mercurial (current: %s); disabling)\n'