Use extras_require[test] instead of tests_require (#28)

This replaces `tests_require` with a pattern that seems to have popped
up since pypa/setuptools#931 and pypa/setuptools#1684. It allows test
dependencies to be installed by adding `[test]` to the end of the
package name.

setup.cfg

@@ -32,7 +32,10 @@ install_requires =
  colcon-core
  coverage
 packages = find:
-tests_require =
+zip_safe = false
+
+[options.extras_require]
+test =
  flake8>=3.6.0
  flake8-blind-except
  flake8-builtins
@@ -47,7 +50,6 @@ tests_require =
  pytest
  pytest-cov
  scspell3k>=2.2
-zip_safe = false
 
 [options.packages.find]
 exclude =