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>