py/lexer: Fix parsing of f'{{'.

Prior to this fix, f'{{' would raise a SyntaxError.

Signed-off-by: Damien George <damien@micropython.org>

py/lexer.c

@@ -408,6 +408,7 @@ static void parse_string_literal(mp_lexer_t *lex, bool is_raw, bool is_fstring)
                     vstr_add_byte(&lex->fstring_args, ',');
                 }
                 vstr_add_byte(&lex->vstr, '{');
+                goto continue_outer;
             }
             #endif
 
@@ -518,6 +519,9 @@ static void parse_string_literal(mp_lexer_t *lex, bool is_raw, bool is_fstring)
         }
     continue_parsing_string_literal:
         next_char(lex);
+        #if MICROPY_PY_FSTRINGS
+    continue_outer:;
+        #endif
     }
 
     // check we got the required end quotes

tests/basics/string_fstring.py

@@ -31,6 +31,8 @@ print(f"a{ {0,1,2}}")
 
 # PEP-0498 specifies that handling of double braces '{{' or '}}' should
 # behave like str.format.
+print(f'{{')
+print(f'}}')
 print(f'{{}}')
 print(f'{{{4*10}}}', '{40}')