Drops Python 2 support

Makes sure the user is warned at install time if using Python 2.

.github/workflows/test_python.yml

@@ -58,3 +58,18 @@ jobs:
         pip install Cython==0.28.6
     - run: buildozer --help
     - run: buildozer init
+
+  Python2:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Setup python
+      uses: actions/setup-python@v2
+      with:
+        python-version: 2.7
+    - uses: actions/checkout@v2
+    - name: Try Python 2 install
+      run: |
+        # we don't want to build to fail with the exit 1 so we catch with "||"
+        python2 -m pip install -e . 2> error.log || echo Failing as expected
+        cat error.log
+        grep "Unsupported Python version" error.log

setup.py

@@ -2,6 +2,7 @@
 Buildozer
 '''
 
+import sys
 from setuptools import setup
 from os.path import dirname, join
 import codecs
@@ -11,6 +12,21 @@ import io
 
 here = os.path.abspath(os.path.dirname(__file__))
 
+CURRENT_PYTHON = sys.version_info[:2]
+REQUIRED_PYTHON = (3, 6)
+
+# This check and everything above must remain compatible with Python 2.7.
+if CURRENT_PYTHON < REQUIRED_PYTHON:
+    sys.stderr.write("""
+==========================
+Unsupported Python version
+==========================
+This version of buildozer requires Python {}.{}, but you're trying to
+install it on Python {}.{}.
+""".format(*(REQUIRED_PYTHON + CURRENT_PYTHON)))
+    sys.exit(1)
+
+
 
 def find_version(*file_paths):
     # Open in Latin-1 so that we avoid encoding errors.
@@ -52,14 +68,10 @@ setup(
     classifiers=[
         'Development Status :: 4 - Beta', 'Intended Audience :: Developers',
         'Topic :: Software Development :: Build Tools',
-        'Programming Language :: Python :: 2',
-        'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.1',
-        'Programming Language :: Python :: 3.2',
-        'Programming Language :: Python :: 3.3',
-        'Programming Language :: Python :: 3.4',
-        'Programming Language :: Python :: 3.5'
+        'Programming Language :: Python :: 3.6'
+        'Programming Language :: Python :: 3.7'
+        'Programming Language :: Python :: 3.8'
     ],
     entry_points={
         'console_scripts': [

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py27,py3
+envlist = pep8,py3
 
 [testenv]
 deps =