fix: doc: Update supported Python versions

INSTALL.rst

@@ -22,7 +22,7 @@ System requirements
 --------------------
 
 - Tor (last stable version is recommended)
-- Python 3 (>= 3.5)
+- Python 3 (>= 3.6)
 - virtualenv_ (while there is not ``stem`` release > 1.6.0, it is
   recommended to install the required python dependencies in a virtualenv)
 

setup.py

@@ -51,8 +51,11 @@ setup(
         'Intended Audience :: Developers',
         'Intended Audience :: System Administrators',
         'Operating System :: OS Independent',
-        'Programming Language :: Python :: 3.5',
+        'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
+        'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         'Topic :: System :: Networking',
     ],
     packages=find_packages(),