Excluded pytest 6.0.0 from test dependencies to fix not-callable issue

Details:

* pytest 6.0.0 causes pylint to report many 'not-callable' issues for its
  pytest.mark.*() functions. Mitigated this by excluding pytest 6.0.0
  from the test dependencies. Just excluding it instead of pinning it
  to <6.0.0 will pull in the next update of pytest to see whether the
  issue was fixed. See pytest issue
  pytest-dev/pytest#7558.

Signed-off-by: Andreas Maier <andreas.r.maier@gmx.de>

test-requirements.txt

@@ -15,9 +15,10 @@ packaging>=17.0
 funcsigs>=1.0.2; python_version < '3.3'
 # pytest 5.0.0 has removed support for Python < 3.5
 # pytest 4.3.1 solves an issue on Python 3 with minimum package levels
+# pytest 6.0.0 causes pylint to report "not-callable" issues
 pytest>=4.3.1,<5.0.0; python_version < '3.5'
-pytest>=4.3.1; python_version >= '3.5' and python_version <= '3.6'
-pytest>=4.4.0; python_version >= '3.7'
+pytest>=4.3.1,!=6.0.0; python_version >= '3.5' and python_version <= '3.6'
+pytest>=4.4.0,!=6.0.0; python_version >= '3.7'
 testfixtures>=6.9.0
 # colorama 0.4.0 removed support for Python 3.4
 colorama>=0.3.9,<0.4.0; python_version <= '3.4'