Use tox feature "extras" to install extras_require dependencies

Avoids duplicating the list of dependencies. They will now always stay
in sync with the setuptools definition. The feature was added in tox
2.4 (2016-10-12), so added a "minversion" to the configuration.

https://tox.readthedocs.io/en/latest/config.html#conf-extras

> extras
>
> A list of “extras” to be installed with the sdist or develop install.
> For example, extras = testing is equivalent to [testing] in a pip
> install command.

1 files changed, 4 insertions, 3 deletions

diff --git a/tox.ini b/tox.ini
index 464c784c..c2558e4e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,4 +1,5 @@
 [tox]
+minversion = 2.4
 envlist =
     isort-check,
     lint,
@@ -7,9 +8,9 @@ envlist =
 [testenv]
 deps =
     pytest
-    pip
-    pipreqs
-    requirementslib
+extras =
+    pipfile
+    requirements
 commands = py.test test_isort.py {posargs}
 
 [testenv:isort-check]