requirements-test: Require newer pytest

Some older versions of pytest don't seem to support named
attributes (`out` and `err`) for the return value of
`capsys.readouterr`, which we depend on in the tests.

Author: suutari-ai

requirements-test.txt

@@ -1,4 +1,4 @@
 mock>=2.0.0
-pytest
+pytest>=3.6.2
 pytest-cov
 -e .[MySQL]