setup.py: Add more metadata useful for PyPI

- setup.py: List useful links using `project_urls`

- setup.py: Specify minumum python vresion using `python_requires`

  `python_requires` is the new way of specifying requirements for python
  version, as documented @
  https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires.

  This will also make this requirement appear nicely on the PyPI project
  page.

- setup.py: Add project classifiers

  PyPI will use these classifiers to categorize projects while searching
  or browsing. Full list of classifiers can be found at
  https://pypi.org/classifiers.

setup.py

@@ -242,11 +242,28 @@ setup(name='BuildStream',
 
       author='BuildStream Developers',
       author_email='buildstream-list@gnome.org',
+      classifiers=[
+          'Environment :: Console',
+          'Intended Audience :: Developers',
+          'License :: OSI Approved :: GNU Lesser General Public License v2 or later (LGPLv2+)',
+          'Operating System :: POSIX',
+          'Programming Language :: Python :: 3',
+          'Programming Language :: Python :: 3.5',
+          'Programming Language :: Python :: 3.6',
+          'Programming Language :: Python :: 3.7',
+          'Topic :: Software Development :: Build Tools'
+      ],
       description='A framework for modelling build pipelines in YAML',
       license='LGPL',
       long_description=long_description,
       long_description_content_type='text/x-rst; charset=UTF-8',
       url='https://gitlab.com/BuildStream/buildstream',
+      project_urls={
+          'Documentation': 'https://buildstream.gitlab.io/buildstream/',
+          'Tracker': 'https://gitlab.com/BuildStream/buildstream/issues',
+          'Mailing List': 'https://mail.gnome.org/mailman/listinfo/buildstream-list'
+      },
+      python_requires='>=3.5',
       packages=find_packages(exclude=('tests', 'tests.*')),
       package_data={'buildstream': ['plugins/*/*.py', 'plugins/*/*.yaml',
                                     'data/*.yaml', 'data/*.sh.in']},