Locate VERSION file relative to the version.py module

In case this is run from somewhere else than the libjuju top directory
(e.g. sphinx).

juju/version.py

@@ -1,6 +1,7 @@
 # Copyright 2023 Canonical Ltd.
 # Licensed under the Apache V2, see LICENCE file for details.
 
+import pathlib
 import re
 
 LTS_RELEASES = ["jammy", "focal", "bionic", "xenial", "trusty", "precise"]
@@ -14,5 +15,5 @@
 # to update the client to the latest.
 # However, for any CLIENT_VERSION > juju-controller-version, a "client incompatible
 # with server" will be returned by the juju controller.
-VERSION_FILE_PATH = './VERSION'
+VERSION_FILE_PATH = pathlib.Path(__file__).parent.parent / 'VERSION'
 CLIENT_VERSION = re.search(r'\d+\.\d+\.\d+', open(VERSION_FILE_PATH).read().strip()).group()