Drop support for old Python versions (#149)

Specifically, drop support for Python versions older than 3.7, including
Python 2.x.

All of those versions are well beyond their official end-of-life, and
given this is a security-related project, it's reasonable to expect
users to run it in a modern Python environment.

.travis.yml

@@ -3,11 +3,10 @@ branches:
     - "master"
 language: python
 python:
-  - "2.7"
-  - "3.5"
-  - "3.6"
   - "3.7"
   - "3.8"
+  - "3.9"
+  - "3.10"
 cache: pip
 install:
   - pip install tox-travis

setup.py

@@ -18,7 +18,7 @@ setup(
         ],
     },
     include_package_data=True,
-    python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, <4',
+    python_requires='>=3.7, <4',
     classifiers=[
         'Development Status :: 5 - Production/Stable',
         'Intended Audience :: Developers',
@@ -26,13 +26,11 @@ setup(
         'License :: OSI Approved :: MIT License',
         'Operating System :: OS Independent',
         'Programming Language :: Python',
-        'Programming Language :: Python :: 2',
-        'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.5',
-        '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',
         'Topic :: Software Development :: Libraries :: Python Modules',
     ],
     zip_safe=False,

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27, py34, py35, py36, py37, py38, flake8
+envlist = py37, py38, py39, py310, flake8
 
 [testenv]
 setenv =