SCons: Fix Godot detection in custom modules logic
SCons: Fix Godot detection in custom modules logic

`exec()` was not a good idea as it assumes a certain type of `version.py` file
similar to Godot's own file, which is not always a reliable assumption (see
https://github.com/godotengine/godot/pull/43057#issuecomment-777632900).

Also restores Python 2 support for the 3.2 branch.

honix/godot,josempans/godot,Shockblast/godot,DmitriySalnikov/godot,sanikoyes/godot,BastiaanOlij/godot,honix/godot,pkowal1982/godot,akien-mga/godot,akien-mga/godot,vkbsb/godot,Faless/godot,godotengine/godot,firefly2442/godot,akien-mga/godot,sanikoyes/godot,sanikoyes/godot,Valentactive/godot,vkbsb/godot,Shockblast/godot,pkowal1982/godot,BastiaanOlij/godot,vkbsb/godot,MarianoGnu/godot,MarianoGnu/godot,vkbsb/godot,Valentactive/godot,josempans/godot,sanikoyes/godot,Zylann/godot,BastiaanOlij/godot,ZuBsPaCe/godot,akien-mga/godot,josempans/godot,honix/godot,Valentactive/godot,firefly2442/godot,honix/godot,Valentactive/godot,Faless/godot,BastiaanOlij/godot,josempans/godot,Shockblast/godot,Zylann/godot,sanikoyes/godot,Faless/godot,vnen/godot,ZuBsPaCe/godot,vnen/godot,BastiaanOlij/godot,ZuBsPaCe/godot,DmitriySalnikov/godot,akien-mga/godot,firefly2442/godot,MarianoGnu/godot,Shockblast/godot,vnen/godot,guilhermefelipecgs/godot,vnen/godot,firefly2442/godot,Shockblast/godot,DmitriySalnikov/godot,BastiaanOlij/godot,sanikoyes/godot,Faless/godot,BastiaanOlij/godot,pkowal1982/godot,pkowal1982/godot,josempans/godot,guilhermefelipecgs/godot,firefly2442/godot,guilhermefelipecgs/godot,MarianoGnu/godot,ZuBsPaCe/godot,Zylann/godot,pkowal1982/godot,Valentactive/godot,firefly2442/godot,pkowal1982/godot,Faless/godot,Zylann/godot,DmitriySalnikov/godot,godotengine/godot,akien-mga/godot,vkbsb/godot,ZuBsPaCe/godot,Zylann/godot,DmitriySalnikov/godot,Valentactive/godot,Shockblast/godot,guilhermefelipecgs/godot,pkowal1982/godot,MarianoGnu/godot,josempans/godot,sanikoyes/godot,MarianoGnu/godot,Shockblast/godot,honix/godot,josempans/godot,vnen/godot,DmitriySalnikov/godot,guilhermefelipecgs/godot,BastiaanOlij/godot,Zylann/godot,Valentactive/godot,josempans/godot,Valentactive/godot,guilhermefelipecgs/godot,Faless/godot,godotengine/godot,Faless/godot,vnen/godot,vkbsb/godot,sanikoyes/godot,akien-mga/godot,MarianoGnu/godot,ZuBsPaCe/godot,godotengine/godot,godotengine/godot,firefly2442/godot,DmitriySalnikov/godot,godotengine/godot,vnen/godot,akien-mga/godot,godotengine/godot,MarianoGnu/godot,honix/godot,guilhermefelipecgs/godot,pkowal1982/godot,ZuBsPaCe/godot,ZuBsPaCe/godot,godotengine/godot,vnen/godot,guilhermefelipecgs/godot,Zylann/godot,vkbsb/godot,Faless/godot,Zylann/godot,vkbsb/godot,firefly2442/godot,Shockblast/godot