vim-patch:77cfc49: runtime(python): highlight ellipsis literals

The ellipsis literal (`...`) can be used in multiple contexts:

- Placeholders:     `class Foo: ...`
- Containers:       `Tuple[int, ...]`
- Assignments:      `x = ...`

This is a trickier pattern to match because we can't rely on keyword
boundaries, so we instead look for exactly three dots (`...`).

This does mean that we will match the `...` portion of `x...x`, which
isn't valid Python syntax, but I think that's an acceptable trade-off
that avoids making this pattern much more complex.

Reference:
- https://docs.python.org/3/library/constants.html#Ellipsis

closes: vim/vim#18107

77cfc49060

Co-authored-by: Jon Parise <jon@indelible.org>

runtime/syntax/python.vim

@@ -313,6 +313,8 @@ if !exists("python_no_builtin_highlight")
   syn match   pythonAttribute	/\.\h\w*/hs=s+1
 	\ contains=ALLBUT,pythonBuiltin,pythonClass,pythonFunction,pythonType,pythonAsync
 	\ transparent
+  " the ellipsis literal `...` can be used in multiple syntactic contexts
+  syn match   pythonEllipsis	"\.\@1<!.\.\.\ze\.\@!" display
 endif
 
 " From the 'Python Library Reference' class hierarchy at the bottom.
@@ -368,10 +370,12 @@ if !exists("python_no_doctest_highlight")
   if !exists("python_no_doctest_code_highlight")
     syn region pythonDoctest
 	  \ start="^\s*>>>\s" end="^\s*$"
-	  \ contained contains=ALLBUT,pythonDoctest,pythonClass,pythonFunction,pythonType,@Spell
+	  \ contained contains=ALLBUT,pythonDoctest,pythonEllipsis,pythonClass,pythonFunction,pythonType,@Spell
     syn region pythonDoctestValue
 	  \ start=+^\s*\%(>>>\s\|\.\.\.\s\|"""\|'''\)\@!\S\++ end="$"
-	  \ contained
+	  \ contained contains=pythonEllipsis
+    syn match pythonEllipsis "\%(^\s*\)\@<!\.\@1<!\zs\.\.\.\ze\.\@!" display
+	  \ contained containedin=pythonDoctest
   else
     syn region pythonDoctest
 	  \ start="^\s*>>>" end="^\s*$"
@@ -414,6 +418,7 @@ if !exists("python_no_number_highlight")
 endif
 if !exists("python_no_builtin_highlight")
   hi def link pythonBuiltin		Function
+  hi def link pythonEllipsis		pythonBuiltin
 endif
 if !exists("python_no_exception_highlight")
   hi def link pythonExceptions		Structure