Set python metadata in setup.cfg to >=3.8

This matches zuul and helps communicate the versions required and
tested.

Change-Id: Ie595c9c570ca22f8f097c4602ef35d8c3dc0da52

setup.cfg

@@ -6,7 +6,7 @@ description-file =
 author = Zuul Team
 author-email = zuul-discuss@lists.zuul-ci.org
 home-page = https://zuul-ci.org/
-python-requires = >= 3.7
+python-requires = >= 3.8
 classifier =
     Intended Audience :: Information Technology
     Intended Audience :: System Administrators
@@ -14,7 +14,6 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9