try fixing linkcheck 403 with custom user_agent

xref: sphinx-doc/sphinx#7369

Author: croth1

src/conf.py

@@ -112,6 +112,7 @@
 todo_include_todos = True
 
 # ---- Options for link validation --------
+user_agent = 'Mozilla/5.0 (X11; Linux x86_64; rv:25.0) Gecko/20100101 Firefox/25.0'
 
 anchor_check_fps = [
     r'https://conda-forge.org/status/#armosxaddition$',