Remove outdated py2py3 example (#6306)

Remove outdated py2py3 example

.pre-commit-config.yaml

@@ -1,4 +1,3 @@
-exclude: doc/en/example/py2py3/test_py2.py
 repos:
 -   repo: https://github.com/psf/black
     rev: 19.10b0

doc/en/example/py2py3/conftest.py

@@ -1,16 +0,0 @@
-import sys
-
-import pytest
-
-py3 = sys.version_info[0] >= 3
-
-
-class DummyCollector(pytest.collect.File):
-    def collect(self):
-        return []
-
-
-def pytest_pycollect_makemodule(path, parent):
-    bn = path.basename
-    if "py3" in bn and not py3 or ("py2" in bn and py3):
-        return DummyCollector(path, parent=parent)

doc/en/example/py2py3/test_py2.py

@@ -1,5 +0,0 @@
-def test_exception_syntax():
-    try:
-        0 / 0
-    except ZeroDivisionError, e:
-        assert e

doc/en/example/py2py3/test_py3.py

@@ -1,5 +0,0 @@
-def test_exception_syntax():
-    try:
-        0 / 0
-    except ZeroDivisionError as e:
-        assert e