gnuplot 0.6: add lower bound on iso8601
Lower bounds tests fail with
```
#=== ERROR while compiling gnuplot.0.6 ========================================#
# context 2.2.0~alpha~dev | linux/x86_64 | ocaml-base-compiler.4.12.1 | file:///home/opam/opam-repository
# path ~/.opam/4.12/.opam-switch/build/gnuplot.0.6
# command ~/.opam/opam-init/hooks/sandbox.sh build dune build -p gnuplot -j 127
# exit-code 1
# env-file ~/.opam/log/gnuplot-7-8c3e4f.env
# output-file ~/.opam/log/gnuplot-7-8c3e4f.out
### output ###
# ocamlc src/.gnuplot.objs/byte/gnuplot.{cmo,cmt} (exit 2)
# (cd _build/default && /home/opam/.opam/4.12/bin/ocamlc.opt -w -40 -g -bin-annot -I src/.gnuplot.objs/byte -I /home/opam/.opam/4.12/lib/ISO8601 -intf-suffix .ml -no-alias-deps -o src/.gnuplot.objs/byte/gnuplot.cmo -c -impl src/gnuplot.ml)
# File "src/gnuplot.ml", line 62, characters 4-32:
# 62 | ISO8601.Permissive.pp_format out fmt d tz
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
# Error: Unbound value ISO8601.Permissive.pp_format
# ocamlopt src/.gnuplot.objs/native/gnuplot.{cmx,o} (exit 2)
# (cd _build/default && /home/opam/.opam/4.12/bin/ocamlopt.opt -w -40 -g -I src/.gnuplot.objs/byte -I src/.gnuplot.objs/native -I /home/opam/.opam/4.12/lib/ISO8601 -intf-suffix .ml -no-alias-deps -o src/.gnuplot.objs/native/gnuplot.cmx -c -impl src/gnuplot.ml)
# File "src/gnuplot.ml", line 62, characters 4-32:
# 62 | ISO8601.Permissive.pp_format out fmt d tz
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
# Error: Unbound value ISO8601.Permissive.pp_format
```