Make SWIG aware of PYTHON3 for its bindings and remove threads

This removes the threading from the SWIG interface (it was supposed to
be off by default) and adds support for Python3 bindings when supported
by the system

pcbnew/CMakeLists.txt

@@ -437,8 +437,15 @@ if( KICAD_SCRIPTING )   # Generate pcbnew.py and pcbnew_wrap.cxx using swig
 
     # We deliberately do not use the CMake support for swig here,
     # i.e. swig_add_footprint()) because we want full control.
-
-    set( SWIG_OPTS -python -c++ -outdir ${CMAKE_CURRENT_BINARY_DIR} ${SWIG_FLAGS} )
+    
+    # Avoid threading in SWIG (breaks threads in pcbnew)
+    set( SWIG_OPTS -python -c++ -nothreads  )
+    
+    if( KICAD_SCRIPTING_PYTHON3 )
+        set( SWIG_OPTS ${SWIG_OPTS} -py3 )
+    endif()
+    
+    set( SWIG_OPTS ${SWIG_OPTS} -outdir ${CMAKE_CURRENT_BINARY_DIR} ${SWIG_FLAGS} )
 
     if( EXISTS ${CMAKE_CURRENT_BINARY_DIR}/doxygen-xml )
         set( SWIG_OPTS ${SWIG_OPTS} -DENABLE_DOCSTRINGS_FROM_DOXYGEN )