remove py37

Remove python3.7 from setup.cfg, since Wallaby's python
supported runtimes are python 3.6 and python 3.8[1]:

[1]: https://governance.openstack.org/tc/reference/runtimes/wallaby.html

Change-Id: I26e748d804743ffc3f30cd05347fdd51dedb051c

Author: likui

setup.cfg

@@ -16,7 +16,6 @@ classifier =
     Programming Language :: Python
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
 
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