More improvements to tox set-up

Author: astrofrog

setup.cfg

@@ -29,3 +29,9 @@ install_requires =
 [options.entry_points]
 pytest11 =
     pytest_mpl = pytest_mpl.plugin
+
+[options.extras_require]
+test =
+    pytest-cov
+    matplotlib
+    pillow

tox.ini

@@ -16,9 +16,11 @@ deps =
     mpl30: matplotlib==3.0.*
     mpl31: matplotlib==3.1.*
     mpldev: git+https://github.com/matplotlib/matplotlib.git#egg=matplotlib
+extras =
+    test
 commands =
     pip freeze
-    cov: pytest {toxinidir}/tests --cov pytest_mpl --cov-config={toxinidir}/setup.cfg {posargs}
+    pytest {toxinidir}/tests --cov pytest_mpl --cov-config={toxinidir}/setup.cfg {posargs}
 
 [testenv:codestyle]
 skip_install = true