Add Python 3 to CI

• Add a build and a test job using LCG 96python3
• Build on top of !545 (closed) by @apearce, rebasing and adapting where needed
• All tests passing with both python2 and python3!
• Supersedes !545 (closed) and !864 (closed)

Closes #56 (closed)

Gaudi/python/Gaudi/CommonGaudiConfigurables.py

@@ -59,4 +59,10 @@ for new in aliases:
 _gbl.update(aliases)
 __all__.extend(aliases)
 # remove temporaries
-del _gbl, new
+del _gbl
+# The `new` var is only scoped in its `for` loop in Python 3, so we only need
+# to 'worry' about cleanup in Python 2
+try:
+    del new
+except NameError:
+    pass