[CI] Changed tested python version to 3.7 and 3.10

We cannot add python 3.6 because there is no precompiled version of it to used.
Therefor, will be using 3.7 as the minimum version in CI.
In addition, dropped version limits from pytest.

requirements-tests.txt

@@ -1,4 +1,4 @@
-pytest != 5.2.3, < 5.4
+pytest
 pytest-twisted
 pytest-cov
 mock