tests: run test-check-py3-compat only in pure python mode

This test was added few years ago to detect syntax and import related warnings
and kickstart the python-3 porting efforts. It tries to parse the ast and import
the module.

When using in strict modes like `HGMODULEPOLICY=rust` or `HGMODULEPOLICY=c`, the
test fails. Let's run the test only on pure python mode.

We now have full test suite being run with python 3 and some people even use hg
with python 3 in production (like me).

tests/test-check-py3-compat.t

@@ -1,4 +1,4 b''
-#require test-repo
+#require test-repo pure
 
   $ . "$TESTDIR/helpers-testrepo.sh"
   $ cd "$TESTDIR"/..