Update Python version support (#1645)

* Update Python version support

Stop testing on Python 3.7 (end of life)
Explain more clearly in documentation what versions are supported.
Update list of classifiers.

* Correct version mistake in documentation

.github/workflows/test_python.yml

@@ -6,7 +6,6 @@ jobs:
     strategy:
       matrix:
         python:
-           - '3.7'
            - '3.8'
            - '3.9'
            - '3.10'

docs/source/installation.rst

@@ -2,7 +2,8 @@
 Installation
 ============
 
-Buildozer itself doesn't depend on any library Python >= 3.3.
+Buildozer is tested on Python 3.8 and above but may work on
+earlier versions, back to Python 3.3.
 Depending the platform you want to target, you might need more tools installed.
 Buildozer tries to give you hints or tries to install few things for
 you, but it doesn't cover every situation.

setup.py

@@ -13,7 +13,7 @@
 here = os.path.abspath(os.path.dirname(__file__))
 
 CURRENT_PYTHON = sys.version_info[:2]
-REQUIRED_PYTHON = (3, 6)
+REQUIRED_PYTHON = (3, 8)
 
 # This check and everything above must remain compatible with Python 2.7.
 if CURRENT_PYTHON < REQUIRED_PYTHON:
@@ -70,9 +70,11 @@ def find_version(*file_paths):
         'Intended Audience :: Developers',
         'Topic :: Software Development :: Build Tools',
         'Programming Language :: Python :: 3',
-        '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 :: 3.11',
+
     ],
     entry_points={
         'console_scripts': [