Enable use of await in assignment expressions

This tries to fix prompt-toolkit#447 and some other bugs concerning expressions, like
for instance `for` loops. Which cannot contain awaitables otherwise.

Author: baldulin

ptpython/repl.py

@@ -271,9 +271,14 @@ async def eval_async(self, line: str) -> object:
                 self._store_eval_result(result)
                 return result
 
-            # If not a valid `eval` expression, run using `exec` instead.
+            # If not a valid `eval` expression, compile as `exec` expression
+            # but still run with eval to get an awaitable in case of a
+            # awaitable expression.
             code = self._compile_with_flags(line, "exec")
-            exec(code, self.get_globals(), self.get_locals())
+            result = eval(code, self.get_globals(), self.get_locals())
+
+            if _has_coroutine_flag(code):
+                result = await result
 
         return None