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.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

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