xml.dom.minidom: add missing type annotation of Element.getAttribute (#8350)

Author: tetsuok

stdlib/xml/dom/minidom.pyi

@@ -127,7 +127,7 @@ class Element(Node):
         self, tagName, namespaceURI: str | None = ..., prefix: Any | None = ..., localName: Any | None = ...
     ) -> None: ...
     def unlink(self) -> None: ...
-    def getAttribute(self, attname): ...
+    def getAttribute(self, attname: str) -> str: ...
     def getAttributeNS(self, namespaceURI: str, localName): ...
     def setAttribute(self, attname, value) -> None: ...
     def setAttributeNS(self, namespaceURI: str, qualifiedName: str, value) -> None: ...