Upgrade pytest and pytest-cov (#18542)

* Upgrade pytest to 4.0.0

* Upgrade pytest-cov to 2.6.0

requirements_test.txt

@@ -10,8 +10,8 @@ mypy==0.641
 pydocstyle==2.1.1
 pylint==2.1.1
 pytest-aiohttp==0.3.0
-pytest-cov==2.5.1
+pytest-cov==2.6.0
 pytest-sugar==0.9.2
 pytest-timeout==1.3.2
-pytest==3.10.0
+pytest==4.0.0
 requests_mock==1.5.2

requirements_test_all.txt

@@ -11,10 +11,10 @@ mypy==0.641
 pydocstyle==2.1.1
 pylint==2.1.1
 pytest-aiohttp==0.3.0
-pytest-cov==2.5.1
+pytest-cov==2.6.0
 pytest-sugar==0.9.2
 pytest-timeout==1.3.2
-pytest==3.10.0
+pytest==4.0.0
 requests_mock==1.5.2