Added correct license info

Author: mrpowerus

setup.cfg

@@ -11,7 +11,7 @@ project_urls =
     Bug Tracker = https://github.com/mrpowerus/salesforce-bulk-python/issues
 classifiers =
     Programming Language :: Python :: 3
-    License :: OSI Approved :: MIT License
+    License :: OSI Approved :: GNU General Public License v3 (GPLv3)
     Operating System :: OS Independent
 
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