Added pyproject extra to install toml package as an optional dependency (#964)

* Add toml package as a dependency

* Make toml an optional dependency via new pyproject extra

CHANGELOG

@@ -27,6 +27,7 @@
 - Rename "unwrapped_line" module to "logical_line."
 - Rename "UnwrappedLine" class to "LogicalLine."
 ### Fixed
+- Added pyproject extra to install toml package as an optional dependency.
 - Enable `BLANK_LINE_BEFORE_NESTED_CLASS_OR_DEF` knob for "pep8" style, so
   method definitions inside a class are surrounded by a single blank line as
   prescribed by PEP8.

setup.py

@@ -77,4 +77,7 @@ with codecs.open('README.rst', 'r', 'utf-8') as fd:
       cmdclass={
           'test': RunTests,
       },
+      extras_require={
+          'pyproject': ['toml'],
+      },
   )