*: Python3.9 switched __file__ to be always absolute

https://bugs.python.org/issue20443

This broke test_defer_excchain_dump because
testprog/golang_test_defer_excchain.txt is prepared with output where
`python file` shows that file name in traceback as it was specified on
the command line, e.g.

    .../pygolang/golang/testprog$ python golang_test_defer_excchain.py
    Traceback (most recent call last):
      File ".../pygolang/golang/__init__.py", line 103, in _
        return f(*argv, **kw)
      File "golang_test_defer_excchain.py", line 42, in main
        raise RuntimeError("err")
    RuntimeError: err

while with py39 it became

    .../pygolang/golang/testprog$ python golang_test_defer_excchain.py
    Traceback (most recent call last):
      File ".../pygolang/golang/__init__.py", line 103, in _
        return f(*argv, **kw)
      File ".../pygolang/golang/testprog/golang_test_defer_excchain.py", line 42, in main
        raise RuntimeError("err")
    RuntimeError: err

(notice the difference related to "line 42")

-> Fix it:

- amend the test to conditionally prefix golang_test_defer_excchain.py
  in expected output with PYGOLANG/golang/testprog/ if it is Python >= 3.9.
- amend `gpython file` to match behaviour of underlying `python file`,
  so that the test passes unconditionally whether it is run by python or
  gpython.

--------

@jerome also says (nexedi/pygolang!13 (comment 122826)):

FYI, buildout (and many zope packages) essentially use doctests for
testing and they had to deal with similar differences in output.
`zope.testing` comes with a doctest checker named "renormalizing" which
normalize output with regular expressions, see for example setup of some
buildout test [here](https://github.com/buildout/buildout/blob/db3d6e2fbf5d7ff2cc4b2507253c7a221cfc3e32/src/zc/buildout/tests.py#L3615-L3651).
We definitely don't need this here for the moment, but maybe one day it
can be useful.

/reviewed-on nexedi/pygolang!13