*: 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 (!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 !13

golang/golang_test.py

@@ -1892,7 +1892,12 @@ def assertDoc(want, got):
     got = got.replace(dir_pygolang,  "PYGOLANG") # /home/x/.../pygolang -> PYGOLANG
     got = got.replace(udir_pygolang, "PYGOLANG") # ~/.../pygolang       -> PYGOLANG
 
-    # ^$ -> <BLANKLINE>
+    # want: process conditionals
+    # PY39(...) -> ... if py39 else ø
+    py39 = sys.version_info >= (3, 9)
+    want = re.sub(r"PY39\((.*)\)", r"\1" if py39 else "", want)
+
+    # want: ^$ -> <BLANKLINE>
     while "\n\n" in want:
         want = want.replace("\n\n", "\n<BLANKLINE>\n")
 

golang/testprog/golang_test_defer_excchain.txt

 Traceback (most recent call last):
   File "PYGOLANG/golang/__init__.py", line ..., in _
     return f(*argv, **kw)
-  File "golang_test_defer_excchain.py", line 42, in main
+  File "PY39(PYGOLANG/golang/testprog/)golang_test_defer_excchain.py", line 42, in main
     raise RuntimeError("err")
 RuntimeError: err
 
@@ -10,7 +10,7 @@ During handling of the above exception, another exception occurred:
 Traceback (most recent call last):
   File "PYGOLANG/golang/__init__.py", line ..., in __exit__
     ...
-  File "golang_test_defer_excchain.py", line 31, in d1
+  File "PY39(PYGOLANG/golang/testprog/)golang_test_defer_excchain.py", line 31, in d1
     raise RuntimeError("d1: aaa")
 RuntimeError: d1: aaa
 
@@ -19,14 +19,14 @@ During handling of the above exception, another exception occurred:
 Traceback (most recent call last):
   File "PYGOLANG/golang/__init__.py", line ..., in __exit__
     ...
-  File "golang_test_defer_excchain.py", line 33, in d2
+  File "PY39(PYGOLANG/golang/testprog/)golang_test_defer_excchain.py", line 33, in d2
     1/0
 ZeroDivisionError: ...
 
 During handling of the above exception, another exception occurred:
 
 Traceback (most recent call last):
-  ... "golang_test_defer_excchain.py", line 45, in <module>
+  ... "PY39(PYGOLANG/golang/testprog/)golang_test_defer_excchain.py", line 45, in <module>
     main()
   ...
   File "PYGOLANG/golang/__init__.py", line ..., in _
@@ -37,6 +37,6 @@ Traceback (most recent call last):
     ...
   File "PYGOLANG/golang/__init__.py", line ..., in __exit__
     ...
-  File "golang_test_defer_excchain.py", line 35, in d3
+  File "PY39(PYGOLANG/golang/testprog/)golang_test_defer_excchain.py", line 35, in d3
     raise RuntimeError("d3: bbb")
 RuntimeError: d3: bbb

gpython/__init__.py

@@ -160,6 +160,10 @@ def pymain(argv, init=None):
         if len(argv) > 0 and argv[0] != '-':
             sys.argv = argv
             filepath = argv[0]
+            # starting from cpython 3.9 __file__ is always absolute
+            # https://bugs.python.org/issue20443
+            if sys.version_info >= (3, 9):
+                filepath = realpath(filepath)
 
             sys.path.insert(0, realpath(dirname(filepath))) # not abspath -> see PySys_SetArgvEx
             def run():