Use SPDX license expression, ignore dist directory (#53)

Signed-off-by: Leander Stephen D'Souza <leanderdsouza1234@gmail.com>
Co-authored-by: Scott K Logan <logans@cottsay.net>

Author: leander-dsouza

.gitignore

@@ -1,2 +1,3 @@
 __pycache__/
 /*.egg-info/
+/dist/

setup.cfg

@@ -13,13 +13,12 @@ classifiers =
     Development Status :: 3 - Alpha
     Environment :: Plugins
     Intended Audience :: Developers
-    License :: OSI Approved :: Apache Software License
     Operating System :: MacOS
     Operating System :: Microsoft :: Windows
     Operating System :: POSIX
     Programming Language :: Python
     Topic :: Software Development :: Build Tools
-license = Apache License, Version 2.0
+license = Apache-2.0
 description = Extension for colcon to read CLI mixins from files.
 long_description = file: README.rst
 keywords = colcon