typing: set no_implicit_reexport

In Python, if module A defines a name `name`, and module B does `import
name from A`, then another module C can `import name from B`.

Sometimes it is intentional -- module B is meant to "reexport" `name`.
But sometimes it is just confusion/inconsistency on where `name` should
be imported from.

mypy has a flag `--no-implicit-reexport` which puts some order into
this. A name can only be imported from a module if

1. The module defines the name
2. The module's `__all__` includes the name
3. The module imports the name as `from ... import .. as name`.

This flag is included in mypy's `--strict` flag.

I like this flag, but I realize it is a bit controversial, and in
particular item 3 above is a bit unfriendly to contributors who don't
know about it. So I didn't intend to add it to pytest.

But while investigating issue 7589 I came upon mypy issue 8754 which
causes `--no-implicit-reexport` to leak into installed libraries and
causes some unexpected typing differences *in pytest* if the user uses
this flag.

Since the diff mostly makes sense, let's just conform to it.

testing/python/collect.py

@@ -1018,7 +1018,7 @@ class TestTracebackCutting:
         See: https://bitbucket.org/pytest-dev/py/issues/71
         This fixes #995.
         """
-        from _pytest.python import filter_traceback
+        from _pytest._code import filter_traceback
 
         try:
             ns = {}  # type: Dict[str, Any]
@@ -1038,7 +1038,7 @@ class TestTracebackCutting:
         In this case, one of the files in the traceback no longer exists.
         This fixes #1133.
         """
-        from _pytest.python import filter_traceback
+        from _pytest._code import filter_traceback
 
         testdir.syspathinsert()
         testdir.makepyfile(