Add PyMySQL to test requirements and pin pylint < 4.0.0

The functional tests started failing with "backend
'mysql' unavailable" after Neutron change [0].

Projects consuming Neutron's functional test infrastructure
now need to explicitly declare PyMySQL in their own
test requirements to enable MySQL backend connectivity
for functional tests.

This patch also pins pylint to < 4.0.0 to avoid compatibility
issues as done in other OpenStack projects [1].
More details in [2].

[0] https://review.opendev.org/c/openstack/neutron/+/952258
[1] https://review.opendev.org/c/openstack/octavia/+/964892
[2] https://github.com/pylint-dev/pylint/issues/10669

Depends-on: https://review.opendev.org/c/openstack/requirements/+/965107
Change-Id: I16f325c265e22c3e73abd884cc336a2e27d69bec
Signed-off-by: Fernando Royo <froyo@redhat.com>

test-requirements.txt

@@ -5,9 +5,10 @@ coverage!=4.4,>=4.0 # Apache-2.0
 flake8-import-order>=0.18.0,<0.19.0 # LGPLv3
 oslotest>=3.2.0 # Apache-2.0
 stestr>=1.0.0 # Apache-2.0
-pylint>=2.6.0,!=4.0.0,!=4.0.1 # GPLv2
+pylint>=2.6.0,<4.0.0 # GPLv2
 testresources>=2.0.0 # Apache-2.0/BSD
 testscenarios>=0.4 # Apache-2.0/BSD
 WebTest>=2.0.27 # MIT
 testtools>=2.2.0 # MIT
+PyMySQL>=0.7.6 # MIT License
 neutron>=23.0.0.0b3 # Apache-2.0

tox.ini

@@ -60,6 +60,9 @@ commands = false
 setenv = {[testenv:functional]setenv}
          {[testenv:dsvm]setenv}
 deps = {[testenv:functional]deps}
+passenv =
+  {[testenv]passenv}
+  OS_TEST_DBAPI_ADMIN_CONNECTION
 commands =
   stestr run --isolated {posargs}