Fix testing within virtualenv on windows

Updates "pytest-virtualenv" minimum required version to 1.2.5
to include man-group/pytest-plugins#37

Author: jcfr

requirements-dev.txt

@@ -5,6 +5,6 @@ flake8>=3.0.4
 pytest>=3.0.3
 pytest-cov>=2.4.0
 pytest-runner>=2.9
-pytest-virtualenv>=1.2.4
+pytest-virtualenv>=1.2.5
 scikit-build>=0.4.0
 virtualenv>=15.0.3