Update versions for pytest, xdist and attrs in test-requirements.txt (#5399)

This fixes Travis-CI.

Author: gvanrossum

test-requirements.txt

@@ -1,10 +1,11 @@
+attrs>=18.0
 flake8
 flake8-bugbear; python_version >= '3.5'
 flake8-pyi; python_version >= '3.6'
 lxml==4.2.3
 psutil==5.4.0
-pytest>=3.0
-pytest-xdist>=1.18
+pytest>=3.4
+pytest-xdist>=1.22
 pytest-cov>=2.4.0
 typed-ast>=1.1.0,<1.2.0
 typing>=3.5.2; python_version < '3.5'