bpo-30529: Fix errors for invalid whitespaces in f-string subexpressions. (#1888)

'invalid character in identifier' now is raised instead of
'f-string: empty expression not allowed' if a subexpression contains
only whitespaces and they are not accepted by Python parser.

Lib/test/test_fstring.py

@@ -280,6 +280,10 @@ f'{a * x()}'"""
                              "f'{10:{ }}'",
                              "f' { } '",
 
+                             # The Python parser ignores also the following
+                             # whitespace characters in additional to a space.
+                             "f'''{\t\f\r\n}'''",
+
                              # Catch the empty expression before the
                              #  invalid conversion.
                              "f'{!x}'",
@@ -300,6 +304,12 @@ f'{a * x()}'"""
                              "f'{:x'",
                              ])
 
+        # Different error message is raised for other whitespace characters.
+        self.assertAllRaise(SyntaxError, 'invalid character in identifier',
+                            ["f'''{\xa0}'''",
+                             "\xa0",
+                             ])
+
     def test_parens_in_expressions(self):
         self.assertEqual(f'{3,}', '(3,)')