Adjust test requirements

requirements/test.txt

@@ -1,6 +1,6 @@
-pytest >= 4.4.1
+pytest >= 5.0.0
 pytest-helpers-namespace >= 2019.1.8
-pytest-xdist == 1.28.0
+pytest-xdist >= 1.29.0  # For DumpError fix
 pytest-cov >= 2.6.1
 python-xmp-toolkit   # requires apt-get install libexempi3
                      # or brew install exempi