Commit b073f6df (time: Move/Port timers to C++/Pyx nogil) added C++
level PyFunc as a way to call Python function from under nogil code.
We will soon need this functionality for sync.WorkGroup too, so move it
into shared place.

We move into regular library - not a pyx module (e.g. golang.pyx) -
because for pyx modules linking is done at import time by specially
generated code, while we need the linker support to automatically
resolve e.g. calls to PyFunc::operator() from inside std::function.