patman: locate README.rst via importlib

Rationale: this is more robust than assumptions about the file
hierarchy layout of the installation of patman, for example on non
file-hierarchy standard (FHS) systems such as Guix System or Nix OS.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@savoirfairelinux.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/main.py b/tools/patman/main.py
index 8067a28..2a2a7ea 100755
--- a/tools/patman/main.py
+++ b/tools/patman/main.py
@@ -7,6 +7,7 @@
 """See README for more information"""
 
 from argparse import ArgumentParser
+import importlib.resources
 import os
 import re
 import sys
@@ -160,11 +161,8 @@
         fd.close()
 
     elif args.full_help:
-        tools.print_full_help(
-            os.path.join(os.path.dirname(os.path.realpath(sys.argv[0])),
-                         'README.rst')
-        )
-
+        with importlib.resources.path('patman', 'README.rst') as readme:
+            tools.print_full_help(str(readme))
     else:
         # If we are not processing tags, no need to warning about bad ones
         if not args.process_tags: