Run unittests with Python 3 if it is available.

The check.sh script always runs the Python unittests with 'python', but
if 'python3' is in the path, run it with that too.

Fix a Python 3 compat issue and avoid passing None to max() and min().
Use an explicit intersect() function instead to intersect intervals.

lib/cretonne/meta/check.sh

@@ -18,5 +18,14 @@ runif() {
 #
 # Install pylint with 'pip install pylint'.
 runif pylint --py3k --reports=no -- *.py cretonne isa
+
+# Then run the unit tests again with Python 3.
+# We get deprecation warnings about assertRaisesRegexp which was renamed in
+# Python 3, but there doesn't seem to be an easy workaround.
+runif python3 -Wignore:Deprecation -m unittest discover
+
+# Style linting.
 runif flake8 .
+
+# Type checking.
 runif mypy --py2 build.py