requirements: Address pytest regression installing pytest<5.4.0

The version 5.4.0 of pytest introduced a regression preventing pytest-sugar
from working (see [1] and [2])

It has been fixed in pytest (see [3]) and waiting a new release of
pytest is published, this commit updates requirements.

[1] Teemu/pytest-sugar#187
[2] amueller#535 (comment)
[3] pytest-dev/pytest#6986

requirements-dev.txt

@@ -2,7 +2,7 @@ codecov
 coverage
 flake8
 mock
-pytest
+pytest<5.4.0
 pytest-cov
 pytest-sugar
 setuptools>=28.0.0