[tests] Bump pytest dependency versions.
flaky==3.7.0
psutil==5.8.0 # Implicit dependency of pytest-xdist
-pytest==6.1.0
-pytest-benchmark==3.2.3
-pytest-cov==2.11.1
-pytest-mock==3.6.0
-pytest-shard==0.1.1
+pytest==6.2.5
+pytest-benchmark==3.4.1
+pytest-cov==2.12.1
+pytest-mock==3.6.1
+pytest-shard==0.1.2
pytest-stress==1.0.1
pytest-sugar==0.9.4
pytest-timeout==1.4.2
-pytest-xdist==2.2.1
+pytest-xdist==2.4.0
