scmposix: don't subscript IOError

This warning disabling has been in place since late 2019 in 667f56d73ceb.  We
should have had some py3 support at the time, but both pytype complains and
subscripting a real FileNotFoundError generated in `hg debugshell` crashed, so
maybe this fixes a problem.  It looks like all other instances of subscripting
exceptions have been replaced (at least as far as greping for `== errno.`
revealed).

mercurial/scmposix.py

@@ -88,7 +88,7 b' def termsize(ui):'
         except ValueError:
             pass
         except IOError as e:
-            if e[0] == errno.EINVAL:  # pytype: disable=unsupported-operands
+            if e.errno == errno.EINVAL:
                 pass
             else:
                 raise