Default to running tests with Python 3

Specify PY2=1 to use Python 2.

See #1779
Closes #2235

Examples/Makefile.in

@@ -309,7 +309,7 @@ perl5_clean:
 PYTHON_FLAGS =
 
 # Make sure these locate your Python installation
-ifeq (,$(PY3))
+ifneq (,$(PY2))
   PYTHON_INCLUDE= $(DEFS) @PYINCLUDE@
   PYTHON_LIB    = @PYLIB@
   PYTHON        = @PYTHON@ $(PYTHON_FLAGS)
@@ -320,7 +320,7 @@ else
 endif
 
 # Extra Python specific linking options
-ifeq (,$(PY3))
+ifneq (,$(PY2))
   PYTHON_DLNK   = @PYTHONDYNAMICLINKING@
   PYTHON_LINK   = @PYLINK@
 else
@@ -330,7 +330,7 @@ endif
 PYTHON_SO     = @PYTHON_SO@
 
 # SWIG option for Python3
-ifeq (,$(PY3))
+ifneq (,$(PY2))
   SWIGOPTPY3 =
 else
   SWIGOPTPY3 = -py3