Add 'check-pypy' rule to Makefile.

Makefile

@@ -28,6 +28,9 @@ check:: build
 check-nocompat:: build
 	$(RUNTEST) dulwich.tests.nocompat_test_suite
 
+check-pypy:: clean
+	$(MAKE) check-noextensions PYTHON=pypy
+
 check-noextensions:: clean
 	$(RUNTEST) dulwich.tests.test_suite