Avoid using toml as extra (#472)

Avoid pip-compile bug which forces it to produce invalid constraints
files. Installing toml as a direct dependency is enough to avoid
this bug.

Related: jazzband/pip-tools#1300

setup.py

@@ -124,7 +124,8 @@ def run(self):
     ],
     install_requires=[
         'pytest>=4.6',
-        'coverage[toml]>=5.2.1'
+        'coverage>=5.2.1',
+        'toml'
     ],
     python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*',
     extras_require={