Revert "Upgrade Python syntax with pyupgrade --py36-plus"

This reverts commit a0f84c3.

Author: hugovk

app/__init__.py

@@ -9,7 +9,7 @@
     assert version_info >= (3, 7, 0)
 except AssertionError:
     print('Warning Current Python version not supported')
-    print(f'Current Python version: {version_info}')
+    print('Current Python version: {version}'.format(version=version_info))
     exit(1)