Fix CVE-2018-14647:
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-14647
https://bugs.python.org/issue34623

Taken from upstream:
https://github.com/python/cpython/commit/18b20bad75b4ff0486940fba4ec680e96e70f3a2

diff --git a/Include/pyexpat.h b/Include/pyexpat.h
index 5340ef5fa3..3fc5fa54da 100644
--- a/Include/pyexpat.h
+++ b/Include/pyexpat.h
@@ -3,7 +3,7 @@
 
 /* note: you must import expat.h before importing this module! */
 
-#define PyExpat_CAPI_MAGIC  "pyexpat.expat_CAPI 1.0"
+#define PyExpat_CAPI_MAGIC  "pyexpat.expat_CAPI 1.1"
 #define PyExpat_CAPSULE_NAME "pyexpat.expat_CAPI"
 
 struct PyExpat_CAPI 
@@ -43,6 +43,8 @@ struct PyExpat_CAPI
         XML_Parser parser, XML_UnknownEncodingHandler handler,
         void *encodingHandlerData);
     void (*SetUserData)(XML_Parser parser, void *userData);
+    /* might be none for expat < 2.1.0 */
+    int (*SetHashSalt)(XML_Parser parser, unsigned long hash_salt);
     /* always add new stuff to the end! */
 };
 
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