Python Bindings: Switch default configure selection to python3

Previously, if configure was run without the --python switch, or
if that switch selected a version 3 executable, python2 was used.

python3 is now the default. Using --python=python2 will work, but
it isn't recommended.

Closes: #13512

mythtv/configure

@@ -2670,7 +2670,7 @@ ccache="yes"
 distcc="yes"
 qmake_default="qmake"
 qmakespecs_default=""
-python_default="python"
+python_default="python3"
 
 #Bluray
 bdj_type="j2se"