Error with org-agenda with 9.5.3

[Robert Goldman <rpgoldman@sift.net>]

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I just updated org-mode using package.el and got release 9.5.3

Now I get an error in `org-at-timestamp-p` (OATP), which is invoked when 
calling `org-agenda`.

I checked the repository and the problem is in commit 1f617727f1, which 
added to this function the following code block:

```
-  (let* ((regexp (if extended
-                     (if (eq extended 'agenda)
-                         org-element--timestamp-regexp
-		       org-ts-regexp3)
-                   org-ts-regexp2))
+  (let* ((regexp
+          (if extended
+              (if (eq extended 'agenda)
+                  (rx (or (regexp org-ts-regexp3)
+                          (regexp org-element--timestamp-regexp)))
+		org-ts-regexp3)
+            org-ts-regexp2))
```

The issue arises where OATP uses `rx` to process a regex stored in 
`org-ts-regexp3`:


```
(rx (or (regexp org-ts-regexp3) ; here
         (regexp org-element--timestamp-regexp)))
```

This fails because `rx-check` checks the sub-expression `(regexp 
org-ts-regexp3)` to make sure that `org-ts-regexp3` is a string, but 
`org-ts-regexp3` is not evaluated, and the *symbol* `org-ts-regexp3` is 
not a string (it's a symbol), even though it's *value* is a string.

I'm not sure whether this is a bug in org-mode proper (I would have 
thought this would be caught before release) or perhaps org-mode expects 
some version of `rx.el` that I do not have.  Unfortunately, I don't see 
a version number in `rx.el`, only a copyright date.

I am running Emacs 25 (Aquamacs). So, maybe this code requires Emacs 26? 
If so, it would be nice if org-mode 9.5.3 was tagged as incompatible 
with Emacs 25? (It would also be nice if package mode supported 
roll-back, but that's OT here!)


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