Fix loop in too-many-ancestors when an inheritance cycle is inferred (#5062)

* Add regression test for #4972 hang

* Fix loop in too-many-ancestors when an inheritance cycle is inferred

Ref #4972. If a class could be inferred as an ancestor of itself then
the implementation of _get_parents could get caught in an infinite loop,
forever adding itself to to_explore. This could be possible by
monkeypatching an ancestor after class definition like in the following
simplified example:

    class Fake(module.Cls):
        pass

    module.Cls = Fake

Reproducing this is fairly tricky, but this integration test shows the
behaviour as of 1e675abcc2aa931421d7ce300908e734a93fd790:

    #!/bin/sh

    HERE=$(readlink -f .)
    VENV=$HERE/venv-repro
    PIP=$VENV/bin/pip

    python -m venv "$VENV"

    PYLINT=$VENV/bin/pylint

    # assume running in pylint dir
    $PIP install -e .
    $PIP install flask

    mkdir -p repro/flask/
    touch repro/__init__.py
    cat > repro/flask/__init__.py <<'EOF'
    import flask
    import repro.flask  # self-import necessary

    class Fake(flask.Flask):
        pass

    flask.Flask = Fake
    EOF

    echo +++ this is fine
    $PYLINT --rcfile=/dev/null -- repro/flask/
    echo +++ finished

    echo +++ this loops forever
    cd repro/; $PYLINT --rcfile=/dev/null -- flask/
    echo +++ should not reach here

2 files changed, 7 insertions, 0 deletions

diff --git a/tests/regrtest_data/hang/pkg4972/__init__.py b/tests/regrtest_data/hang/pkg4972/__init__.py
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/tests/regrtest_data/hang/pkg4972/__init__.py
diff --git a/tests/regrtest_data/hang/pkg4972/string/__init__.py b/tests/regrtest_data/hang/pkg4972/string/__init__.py
new file mode 100644
index 000000000..4a77f25ac
--- /dev/null
+++ b/tests/regrtest_data/hang/pkg4972/string/__init__.py
@@ -0,0 +1,7 @@
+import string
+import pkg4972.string  # self-import necessary
+
+class Fake(string.Formatter):
+    pass
+
+string.Formatter = Fake