doc: use an absolute path in sys.path

The idea and rationale is similar to https://phab.mercurial-scm.org/D12599
(landed as 1b6e381521c5).

Differential Revision: https://phab.mercurial-scm.org/D12622

doc/check-seclevel.py

@@ -10,7 +10,7 import sys
 
 # import from the live mercurial repo
 os.environ['HGMODULEPOLICY'] = 'py'
-sys.path.insert(0, "..")
+sys.path.insert(0, os.path.abspath(".."))
 from mercurial import demandimport
 
 demandimport.enable()