explicitly allow docstrings in .pxd files

Cython/Compiler/Parsing.py

@@ -1894,6 +1894,8 @@ def p_statement(s, ctx, first_statement = 0):
             return p_pass_statement(s, with_newline=True)
         else:
             if ctx.level in ('c_class_pxd', 'property'):
+                if s.sy == 'BEGIN_STRING':
+                    return p_atom(s)  # allow docstrings and discardable strings
                 s.error("Executable statement not allowed here")
             if s.sy == 'if':
                 return p_if_statement(s)