Rewrite fstring parsing to match CPython 3.6 and the updated PEP 498

- resolve string escapes  only outside of fstring expressions
- reject backslashes inside of fstring expressions
Also make some fstring errors non-fatal to keep parsing.

Cython/Compiler/Parsing.pxd

@@ -68,10 +68,11 @@ cdef p_opt_string_literal(PyrexScanner s, required_type=*)
 cdef bint check_for_non_ascii_characters(unicode string)
 @cython.locals(systr=unicode, is_python3_source=bint, is_raw=bint)
 cdef p_string_literal(PyrexScanner s, kind_override=*)
-@cython.locals(i=Py_ssize_t, size=Py_ssize_t)
-cdef list p_f_string(PyrexScanner s, unicode_value, pos)
+cdef _append_escape_sequence(kind, builder, unicode escape_sequence, PyrexScanner s)
+@cython.locals(i=Py_ssize_t, size=Py_ssize_t, c=Py_UCS4)
+cdef list p_f_string(PyrexScanner s, unicode unicode_value, pos, bint is_raw)
 @cython.locals(i=Py_ssize_t, size=Py_ssize_t, c=Py_UCS4, quote_char=Py_UCS4, NO_CHAR=Py_UCS4)
-cdef tuple p_f_string_expr(PyrexScanner s, unicode_value, pos, Py_ssize_t starting_index)
+cdef tuple p_f_string_expr(PyrexScanner s, unicode_value, pos, Py_ssize_t starting_index, bint is_raw)
 cdef p_list_maker(PyrexScanner s)
 cdef p_comp_iter(PyrexScanner s, body)
 cdef p_comp_for(PyrexScanner s, body)

tests/run/fstring.pyx

@@ -18,6 +18,15 @@ max_long = LONG_MAX
 min_long = LONG_MIN
 
 
+def escaping():
+    """
+    >>> escaping()
+    """
+    assert f'{{{{{"abc"}}}}}{{}}{{' == '{{abc}}{}{'
+    assert f'\x7b}}' == '{}'
+    assert f'{"{{}}"}' == '{{}}'
+
+
 def format2(ab, cd):
     """
     >>> a, b, c = format2(1, 2)