meson: Fix python3 version detection with python3 module
meson: Fix python3 version detection with python3 module

I misread the documentation about the python3 module. The newer python
module puts the language_version in the object returned by
`pymod.find_installation()`, while the python3 module has the same
method in the *module*.

Fixes #985

whot/libratbag,whot/libratbag,libratbag/libratbag,libratbag/libratbag,libratbag/libratbag,whot/libratbag