doc: requirements: Add pyserial

Since 7eaca45, pyserial is required for
building docs because `DeviceAdapter` from the `pytest-twister-harness`
plugin, which is imported by autodoc, now depends on it.

(cherry picked from commit 8c8e2f6)

Original-Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
GitOrigin-RevId: 8c8e2f6
Change-Id: I490a0fe97f243959e15d5b217ee5b341e920562f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5569733
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Reviewed-by: Fabio Baltieri <fabiobaltieri@google.com>
Commit-Queue: Fabio Baltieri <fabiobaltieri@google.com>
Tested-by: Fabio Baltieri <fabiobaltieri@google.com>

doc/requirements.txt

@@ -15,8 +15,9 @@ sphinx-sitemap
 PyYAML>=6.0
 pykwalify
 
-# Used by pytest-twister-harness plugin
+# Used by pytest-twister-harness plugin (imported by autodoc)
 pytest
+pyserial
 
 # Doxygen doxmlparser
 doxmlparser