Replace pkg_resources with importlib

`pkg_resources` has been removed and can be replaced with `importlib` (present since python 3.8)

Author: xgarrido

docs/source/conf.py

@@ -1,8 +1,8 @@
-from pkg_resources import DistributionNotFound, get_distribution
+from importlib.metadata import PackageNotFoundError, version
 
 try:
-    __version__ = get_distribution("pspy").version
-except DistributionNotFound:
+    __version__ = version("pspy")
+except PackageNotFoundError:
     __version__ = "unknown version"
 
 import os