Update pytest and pytest-xdist versions (#462)

This solves issues with pytest-xdist requiring pytest>=3.0

Author: emmatyping

test-requirements.txt

@@ -1,7 +1,7 @@
 flake8; python_version >= '3.6'
 flake8-bugbear; python_version >= '3.6'
 flake8-pyi; python_version >= '3.6'
-pytest>=2.8; python_version >= '3.3'
-pytest-xdist>=1.13; python_version >= '3.3'
+pytest>=3.0; python_version >= '3.3'
+pytest-xdist>=1.18; python_version >= '3.3'
 pytest-cov>=2.4.0; python_version >= '3.3'
 typing>=3.6.1; python_version <= '3.4'