AVRO-4104: Drop support for Python 3.7 (#3276)

Python 3.7 has been EOL since June 2023:
https://devguide.python.org/versions/

It now also breaks the CI because
third-party packages dropped 3.7.

Author: Fokko

lang/py/setup.cfg

@@ -33,7 +33,6 @@ license_files = avro/LICENSE
 license = Apache License 2.0
 classifiers =
     License :: OSI Approved :: Apache Software License
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10

lang/py/tox.ini

@@ -21,7 +21,6 @@ envlist =
     docs
     lint
     typechecks
-    py37
     py38
     py39
     py310