disable a restriction in the f-string format parser that seems arbitrary and difficult to implement

Cython/Compiler/Parsing.py

@@ -1101,8 +1101,6 @@ def p_f_string_expr(s, unicode_value, pos, starting_index):
             c = unicode_value[i]
             if not in_triple_quotes and not in_string:
                 if c == '{':
-                    if nested_depth >= 1:
-                        s.error("nesting of '{' in format specifier is not allowed")
                     nested_depth += 1
                 elif c == '}':
                     if nested_depth > 0:

tests/run/test_fstring.pyx

@@ -302,7 +302,7 @@ f'{a * x()}'"""
         self.assertEqual(f'{10:{"#"}1{0}{"x"}}', '       0xa')
         self.assertEqual(f'{-10:-{"#"}1{0}x}', '      -0xa')
         self.assertEqual(f'{-10:{"-"}#{1}0{"x"}}', '      -0xa')
-#        self.assertEqual(f'{10:#{3 != {4:5} and width}x}', '       0xa')
+        self.assertEqual(f'{10:#{3 != {4:5} and width}x}', '       0xa')
 
         self.assertAllRaise(SyntaxError, "f-string: expecting '}'",
                             ["""f'{"s"!r{":10"}}'""",
@@ -315,10 +315,13 @@ f'{a * x()}'"""
                              "f'{4:{/5}}'",
                              ])
 
-        self.assertAllRaise(SyntaxError, "f-string: expressions nested too deeply",
-                            [# Can't nest format specifiers.
-                             "f'result: {value:{width:{0}}.{precision:1}}'",
-                             ])
+        # CYTHON: The nesting restriction seems rather arbitrary. Ignoring it for now and instead test that it works.
+        if not IS_PY26:
+            self.assertEqual(f'result: {value:{width:{0}}.{precision:1}}', 'result:      12.35')
+        #self.assertAllRaise(SyntaxError, "f-string: expressions nested too deeply",
+        #                    [# Can't nest format specifiers.
+        #                     "f'result: {value:{width:{0}}.{precision:1}}'",
+        #                     ])
 
         self.assertAllRaise(SyntaxError, 'f-string: invalid conversion character',
                             [# No expansion inside conversion or for