mention new support for py38

Author: Kriechi

HISTORY.rst

@@ -11,9 +11,9 @@ API Changes (Backward-Incompatible)
 - Support for Python 3.4 has been removed.
 - Support for Python 3.5 has been removed.
 - Support for PyPy (Python 2.7 compatible) has been removed.
+- Support for Python 3.8 has been added.
 - Receiving DATA before HEADERS now raises a ProtocolError (see https://tools.ietf.org/html/rfc7540#section-8.1)
 
-
 3.2.0 (2020-02-08)
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