doc: requirements: Add anytree requirement

Some changes in zephyr docs require anytree.

Signed-off-by: Sean Madigan <[email protected]>

Author: sean-madigan

doc/requirements.txt

@@ -25,3 +25,4 @@ sphinx_markdown_tables          # |     |         |   X    |         |         |
 sphinxcontrib-mscgen            # |  X  |         |        |         |    X    |      |        |
 sphinxcontrib-plantuml>=0.27    # |     |         |        |         |         |  X   |        |
 west>=1.0.0                     # |     |         |        |         |         |      |   X    |
+anytree                         # |     |         |        |         |         |      |   X    |