bpo-45447: Add syntax highlighting for `.pyi` files in IDLE (GH-28950)

Also add .pyi to the python extensions in the "File-open" and "File-save" dialogues.
Add util.py to contain objects that are used in multiple idlelib modules
and have no dependencies on any of them.

Co-authored-by: E-Paine <63801254+E-Paine@users.noreply.github.com>
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
(cherry picked from commit 50cf4991c49e19f917305dd7b9c71085c11edddb)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/Lib/idlelib/util.py b/Lib/idlelib/util.py
new file mode 100644
index 0000000..5480219
--- /dev/null
+++ b/Lib/idlelib/util.py
@@ -0,0 +1,22 @@
+"""
+Idlelib objects with no external idlelib dependencies
+which are needed in more than one idlelib module.
+
+They are included here because
+    a) they don't particularly belong elsewhere; or
+    b) because inclusion here simplifies the idlelib dependency graph.
+
+TODO:
+    * Python versions (editor and help_about),
+    * tk version and patchlevel (pyshell, help_about, maxos?, editor?),
+    * std streams (pyshell, run),
+    * warning stuff (pyshell, run).
+"""
+from os import path
+
+# .pyw is for Windows; .pyi is for stub files.
+py_extensions = ('.py', '.pyw', '.pyi')  # Order needed for open/save dialogs.
+
+if __name__ == '__main__':
+    from unittest import main
+    main('idlelib.idle_test.test_util', verbosity=2)