Target lang detection in Python source code
It would be *very* helpful when building test cases if we can use logic like the following to exclude some target languages:

```py
if not sys.argv[0].endswith(".kt"):
   do_something_which_breaks_only_on_kotlin()
```
That depends on https://github.com/adsharma/py2many/pull/256 and https://github.com/adsharma/py2many/issues/257

Another approach is the set a special global like `sys.implementation` to various values like `py2many-kotlin`,