test-pylint: stop checking hgext3rd

the hgext3rd "packages" contains third party code  by definition, we should not
check it.

tests/test-check-pylint.t

@@ -15,7 +15,7 Current checks:
   >   --enable=W0102,C0321 \
   >   --reports=no \
   >   --ignore=thirdparty \
-  >   mercurial hgdemandimport hgext hgext3rd | sed 's/\r$//'
+  >   mercurial hgdemandimport hgext | sed 's/\r$//'
   Using config file *fakerc (glob) (?)
    (?)
   ------------------------------------* (glob) (?)