Allow docs to build on Python 3.12 by replacing `pkg_resources` with `importlib.metadata`.

- Fix underline length in README.rst

README.rst

@@ -43,7 +43,7 @@ files.
 .. _`BSD`: https://github.com/collective/icalendar/issues/2
 
 Quick start guide
------------
+-----------------
 
 ``icalendar`` enables you to **create**, **inspect** and **modify**
 calendaring information with Python.

docs/conf.py

@@ -1,5 +1,5 @@
 # icalendar documentation build configuration file
-import pkg_resources
+import importlib.metadata
 import datetime
 import os
 
@@ -28,7 +28,7 @@ master_doc = 'index'
 project = 'icalendar'
 this_year = datetime.date.today().year
 copyright = f'{this_year}, Plone Foundation'
-version = pkg_resources.get_distribution('icalendar').version
+version = importlib.metadata.version('icalendar')
 release = version
 
 exclude_patterns = ['_build', 'lib', 'bin', 'include', 'local']