rename the default translation branch to 3.7

Author: zhsj

Diff for: README.rst

@@ -4,7 +4,7 @@ zh_CN Translation of the Python Documentation
 All translations are done on transifex.
 https://www.transifex.com/python-doc/public/
 
-See branch newest for po files fetched from transifex.
+See branch 3.7 for po files fetched from transifex.
 
 Documentation Contribution Agreement
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