python: simplify PYPI metadata to just supporting Python 3

Prevents the supported versions from becoming out of sync when the wheel
build is updated.

Author: jgriffiths

setup.py

@@ -109,11 +109,7 @@ def call(cmd):
 
         'License :: OSI Approved :: MIT License',
 
-        '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',
+        'Programming Language :: Python :: 3',
     ],
     'keywords': 'Bitcoin wallet BIP32 BIP38 BIP39 secp256k1',
     'project_urls': {