parsexp.v0.11.0 is not compatible with OCaml 4.13

[kit-ty-kate]

Backports #18966 to v0.11.0 as well:

#=== ERROR while compiling parsexp.v0.11.0 ====================================#
# context              2.1.0~rc2 | linux/x86_64 | ocaml-variants.4.13.0+trunk | file:///src
# path                 ~/.opam/4.13/.opam-switch/build/parsexp.v0.11.0
# command              ~/.opam/opam-init/hooks/sandbox.sh build jbuilder build -p parsexp -j 47
# exit-code            1
# env-file             ~/.opam/log/parsexp-19-6f2f73.env
# output-file          ~/.opam/log/parsexp-19-6f2f73.out
### output ###
# File "src/parser_automaton_internal.ml", line 651, characters 8-30:
# 651 |         Sexp (Atom str, stack)
#               ^^^^^^^^^^^^^^^^^^^^^^
# Error: The constructor Sexp expects 0 argument(s),
#        but is applied here to 1 argument(s)

Detected in #18994