add python37 setup.cfg

Add python3.7 from setup.cfg, Wallaby's python
supported runtimes are python 3.7[1]:

[1] http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019648.html

Change-Id: Ia1e29c25df0b3f47b9513b863a6f3866a7e802c3

Author: likui

setup.cfg

@@ -16,6 +16,7 @@ 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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