scmposix: another suppression on IOError subscripting

As before, this will clear up when we move to Python 3-only.

Differential Revision: https://phab.mercurial-scm.org/D7280

mercurial/scmposix.py

@@ -90,7 +90,7 b' def termsize(ui):'
         except ValueError:
             pass
         except IOError as e:
-            if e[0] == errno.EINVAL:
+            if e[0] == errno.EINVAL:  # pytype: disable=unsupported-operands
                 pass
             else:
                 raise