chore: Update project metadata to reflect minimum Python version 3.8 (w…

…eaveworks#652)

The package is no longer compatible with Python 3.7 after starting to
use `typing.Literal` recently.

setup.py

@@ -33,7 +33,6 @@ def local_file(name):
         'Intended Audience :: Developers',
         'Intended Audience :: System Administrators',
         'License :: OSI Approved :: Apache Software License',
-        'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: 3.10',