Cap pycodestyle to be < 2.6.0

The 2.6.0 version introduces some checks that cause failures
with the current code. To avoid that, cap pycodestyle to a
version that had been tested without errors.

In Rocky we already had tests with pycodestyle, but proper listing in
test-requirements.txt was only added in
I33be4f5d4ae48c6bd48d80e3f1185ef8307a2a0c

Conflicts:
	test-requirements.txt

Change-Id: I00a35884b14af3e2cf751c04312c847ecfe658c7
(cherry picked from commit 719cae183a)

test-requirements.txt

@@ -7,6 +7,7 @@ bandit>=1.1.0,<1.5.0 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
 flake8-import-order==0.12 # LGPLv3
+pycodestyle>=2.0.0,<2.6.0 # MIT
 mock>=2.0.0 # BSD
 python-subunit>=1.0.0 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT