build: Move to pyproject.toml over setup.cfg

Convert to the more modern and preferred pyproject.toml configuration
for the project.

- Add the majority of the configuration to pyproject.toml.
- Remove setup.cfg as it is no longer needed.
- Update tox.ini to remove a lingering testenv for Python 2.7 which is
  no longer used.

NOTE: Manifest.in is left as the files it includes are outside of the
package directories.

tox.ini

@@ -13,11 +13,6 @@ commands =
     flake8
     pytest
 
-[testenv:py27]
-deps =
-    configparser
-    {[testenv]deps}
-
 [flake8]
 exclude = .tox,env,build
 max-line-length = 120