fix rst formatting to match towncrier config (#30)

Author: pacrob

HISTORY.rst

@@ -1,18 +1,20 @@
 History
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+=======
+
+.. towncrier release notes start
 
 1.0.3 (2020-10-26)
-==================
+------------------
 * Add base36 and base36 upper encodings
 * Fix issue with base16 encoding
 
 1.0.0 (2018-10-19)
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+------------------
 
 * Re-implement encoding for base32 and base64, as the implementation was buggy
 * Add extensive tests for all encodings
 
 0.1.0 (2017-09-02)
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+------------------
 
 * First release on PyPI.