tox: Replace py.test command with pytest

From pytest 3.0 supported and recommended way is to use of pytest
as the main command instead of py.test.

It's possible that in future py.test will be deprecated
or potentially even removed [1].

---

[1]: pytest-dev/pytest#1629

Signed-off-by: Michal Šoltis <msoltis@redhat.com>

tox.ini

@@ -17,7 +17,7 @@ usedevelop = true
 deps =
     -rrequirements-extras.txt
 commands =
-    py.test \
+    pytest \
       --ignore=tests/integration \
       --cov=cachi2 \
       --cov-config=pyproject.toml \