bpo-29762: More use "raise from None". (#569)

This hides unwanted implementation details from tracebacks.
diff --git a/Lib/xml/dom/xmlbuilder.py b/Lib/xml/dom/xmlbuilder.py
index e9a1536..60a2bc3 100644
--- a/Lib/xml/dom/xmlbuilder.py
+++ b/Lib/xml/dom/xmlbuilder.py
@@ -80,7 +80,7 @@
                 settings = self._settings[(_name_xform(name), state)]
             except KeyError:
                 raise xml.dom.NotSupportedErr(
-                    "unsupported feature: %r" % (name,))
+                    "unsupported feature: %r" % (name,)) from None
             else:
                 for name, value in settings:
                     setattr(self._options, name, value)
diff --git a/Lib/xml/etree/ElementPath.py b/Lib/xml/etree/ElementPath.py
index ab6b79a..361f6d5 100644
--- a/Lib/xml/etree/ElementPath.py
+++ b/Lib/xml/etree/ElementPath.py
@@ -80,7 +80,7 @@
                     raise KeyError
                 yield token[0], "{%s}%s" % (namespaces[prefix], uri)
             except KeyError:
-                raise SyntaxError("prefix %r not found in prefix map" % prefix)
+                raise SyntaxError("prefix %r not found in prefix map" % prefix) from None
         else:
             yield token