Add Python3 testing support

Conditionally runs the tests depending on the availability of Python
versions (because Travis is the worst).
diff --git a/.gitignore b/.gitignore
index 9c9ae5a..a6b34fd 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,8 +4,8 @@
 libs
 objs
 
-# Python virtual environment (pre-3.4 only)
-python2.7_virtual_environment
+# Python virtual environments
+python*_virtual_environment
 
 # gcov coverage data
 coverage