Update setup.py to reflect dropping 3.6 & 3.7 support.

Author: CodeReclaimers

.travis.yml

@@ -22,8 +22,6 @@
         'Intended Audience :: Science/Research',
         'License :: OSI Approved :: BSD License',
         'Operating System :: OS Independent',
-        'Programming Language :: Python :: 3.6',
-        'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: 3.10',