update setup python action to v4

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[auvipy]

py36-django32 run-test: commands[0] | python -W error::DeprecationWarning -W error::PendingDeprecationWarning runtests.py --coverage
Traceback (most recent call last):
File "runtests.py", line 4, in
import pytest
File "/home/runner/work/django-rest-framework/django-rest-framework/.tox/venvs/py36-django32/lib/python3.6/site-packages/pytest/init.py", line 3, in
from . import collect
File "/home/runner/work/django-rest-framework/django-rest-framework/.tox/venvs/py36-django32/lib/python3.6/site-packages/pytest/collect.py", line 8, in
from _pytest.deprecated import PYTEST_COLLECT_MODULE
File "/home/runner/work/django-rest-framework/django-rest-framework/.tox/venvs/py36-django32/lib/python3.6/site-packages/_pytest/deprecated.py", line 13, in
from _pytest.warning_types import PytestDeprecationWarning
File "/home/runner/work/django-rest-framework/django-rest-framework/.tox/venvs/py36-django32/lib/python3.6/site-packages/_pytest/warning_types.py", line 6, in
import attr
File "/home/runner/work/django-rest-framework/django-rest-framework/.tox/venvs/py36-django32/lib/python3.6/site-packages/attr/init.py", line 32, in
DeprecationWarning,
DeprecationWarning: Running attrs on Python 3.6 is deprecated & we intend to drop support soon. If that's a problem for you, please let us know why & we MAY re-evaluate: python-attrs/attrs#993
ERROR: InvocationError for command /home/runner/work/django-rest-framework/django-rest-framework/.tox/venvs/py36-django32/bin/python -W error::DeprecationWarning -W error::PendingDeprecationWarning runtests.py --coverage (exited with code 1)

[auvipy]

#8828