require dune 3.0, use proper mdx stanza

dune

@@ -1,9 +1,5 @@
-(rule
- (alias runtest)
- (deps
-  (:readme README.md))
+
+(mdx
  (package printbox-html)
- (action
-  (progn
-   (run ocaml-mdx test %{readme})
-   (diff? %{readme} %{readme}.corrected))))
+ (libraries printbox printbox-text printbox-html)
+ (files README.md))

dune-project

@@ -1,5 +1,6 @@
-(lang dune 2.0)
+(lang dune 3.0)
 (name printbox)
+(using mdx 0.2)
 (generate_opam_files true)
 (version 0.7)
 (license "BSD-2-Clause")

printbox-html.opam

@@ -12,15 +12,16 @@ license: "BSD-2-Clause"
 homepage: "https://github.com/c-cube/printbox"
 bug-reports: "https://github.com/c-cube/printbox/issues"
 depends: [
-  "dune" {>= "2.0"}
+  "dune" {>= "3.0"}
   "printbox" {= version}
   "printbox-text" {= version & with-test}
   "odoc" {with-test}
   "tyxml" {>= "4.3"}
   "mdx" {>= "1.4" & with-test}
+  "odoc" {with-doc}
 ]
 build: [
-  ["dune" "subst"] {pinned}
+  ["dune" "subst"] {dev}
   [
     "dune"
     "build"

printbox-text.opam

@@ -8,15 +8,16 @@ license: "BSD-2-Clause"
 homepage: "https://github.com/c-cube/printbox"
 bug-reports: "https://github.com/c-cube/printbox/issues"
 depends: [
-  "dune" {>= "2.0"}
+  "dune" {>= "3.0"}
   "printbox" {= version}
   "uutf" {>= "1.0"}
   "uucp" {>= "2.0"}
   "odoc" {with-test}
   "mdx" {>= "1.4" & with-test}
+  "odoc" {with-doc}
 ]
 build: [
-  ["dune" "subst"] {pinned}
+  ["dune" "subst"] {dev}
   [
     "dune"
     "build"

printbox.opam

@@ -10,12 +10,12 @@ tags: ["print" "box" "table" "tree"]
 homepage: "https://github.com/c-cube/printbox"
 bug-reports: "https://github.com/c-cube/printbox/issues"
 depends: [
-  "dune" {>= "2.0"}
+  "dune" {>= "3.0"}
   "ocaml" {>= "4.08"}
   "odoc" {with-doc}
 ]
 build: [
-  ["dune" "subst"] {pinned}
+  ["dune" "subst"] {dev}
   [
     "dune"
     "build"

test/dune

@@ -1,4 +1,7 @@
 
+(env
+  (_ (flags :standard -warn-error -a)))
+
 (test
  (name test_ann_0_3)
  (modules test_ann_0_3)

test/test1.ml

@@ -75,7 +75,7 @@ module Box_in = struct
   let () = print_endline @@ PrintBox_text.to_string b
 end
 
-let b =
+let _b =
   let open PrintBox in
   frame @@ record [
     ("subject", text_with_style Style.bold "announce: printbox 0.3");