[3.6] bpo-30682: Removed a too-strict assertion that failed for certain f-strings. (GH-2232) (#2242)

This caused a segfault on eval("f'\\\n'") and eval("f'\\\r'") in debug build..
(cherry picked from commit 11e97f2f80bf65cc828c127eafc95229df35d403)
3 files changed