Replace os.path.join by f-strings with "/" separator

Signed-off-by: Simon Deziel <simon.deziel@canonical.com>

src/charm.py

@@ -5,7 +5,6 @@
 """A Juju charm for Prometheus on Kubernetes."""
 import hashlib
 import logging
-import os
 import re
 import socket
 from typing import Dict, Optional, cast
@@ -472,7 +471,7 @@ def _push_alert_rules(self, alerts):
         """
         for topology_identifier, rules_file in alerts.items():
             filename = f"juju_{topology_identifier}.rules"
-            path = os.path.join(RULES_DIR, filename)
+            path = f"{RULES_DIR}/{filename}"
 
             rules = yaml.safe_dump(rules_file)
 
@@ -689,7 +688,7 @@ def _generate_prometheus_config(self) -> bool:
         """
         prometheus_config = {
             "global": self._prometheus_global_config(),
-            "rule_files": [os.path.join(RULES_DIR, "juju_*.rules")],
+            "rule_files": [f"{RULES_DIR}/juju_*.rules"],
             "scrape_configs": [],
         }