Update pytest requirement from ~=3.10.1 to ~=4.0.1

Updates the requirements on [pytest](https://github.com/pytest-dev/pytest) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest/commits/4.0.1)

Signed-off-by: dependabot[bot] <support@dependabot.com>

dev-requirements.txt

@@ -1,7 +1,7 @@
 coverage ~=4.5.2
 doctr ~=1.7.4
 pyflakes ~=2.0.0
-pytest ~=3.10.1
+pytest ~=4.0.1
 vcrpy ~=2.0.1
 doctr ~=1.7.4
 ipython ~=6.5.0