move towards 3.7 from 3.8

see https://github.com/collective/icalendar/pull/366/files

CHANGES.rst

@@ -7,7 +7,7 @@ Changelog
 
 Breaking changes:
 
-- Require Python 3.8 as minimum Python version.  [maurits]
+- Require Python 3.7 as minimum Python version.  [maurits] [niccokunzmann]
 
 Bug fixes:
 

setup.py

@@ -36,6 +36,7 @@ setuptools.setup(
         'License :: OSI Approved :: BSD License',
         'Operating System :: OS Independent',
         'Programming Language :: Python',
+        'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: 3.10',

tox.ini

@@ -1,6 +1,6 @@
 # to run for a specific environment, use ``tox -e ENVNAME``
 [tox]
-envlist = py38,py39,py310,pypy3,docs
+envlist = py37,py38,py39,py310,pypy3,docs
 # Note: the 'docs' env creates a 'build' directory which may interfere in strange ways
 # with the other environments.  You might see this when you run the tests in parallel.
 # See https://github.com/collective/icalendar/pull/359#issuecomment-1214150269