Clean up install vs. build requirements (#34)

- Exclude `wheel` and `setuptools` from package dependencies
- Exclude `packaging` from build dependencies; however, retain it as a package
  dependency since it is a valid runtime dependency

pyproject.toml

@@ -1,3 +1,3 @@
 [build-system]
-requires = [ "wheel", "setuptools", "packaging" ]
+requires = [ "wheel", "setuptools" ]
 build-backend = "setuptools.build_meta"

setup.py

@@ -37,7 +37,7 @@
       long_description = readme_text,
       ext_modules      = [mgrs],
       packages      = ['mgrs'],
-      install_requires = ['setuptools', 'wheel','packaging'],
+      install_requires = ['packaging'],
       test_suite = 'tests.test_suite',
       zip_safe = False,
       classifiers   = [