prefer more generic Mapping over dict type (#10)

* prefer more generic Mapping over dict type

typically it's better to specify more generic types for input parameters,
and specific types our return values. In this case changing headers
param from dict to Mapping should resolve typing issues when eg.
starlette.requests.headers.Headers instance is passed to
get_principal_from_headers()

* collections.abc.Mapping instead of deprecated typing.Mapping

audit_log/headers.py

@@ -1,4 +1,5 @@
 import re
+from collections.abc import Mapping
 from dataclasses import dataclass
 from urllib.parse import urlparse
 
@@ -65,14 +66,14 @@ def parse_spiffe(xfcc_header: str) -> ParsedSPIFFE:
 
 
 def get_principal_from_headers(
-    headers: dict[str, str],
+    headers: Mapping[str, str],
 ) -> Principal:
     """Get principal from headers, supports mTLS, headers set in Istio, and JWTs.
 
     Note: Do not use this to handle your auth, it expects auth to already be handled elsewhere and this is just to help get principals.
 
     Args:
-        headers (dict[str, str]): Headers with all keys lowercase
+        headers (Mapping[str, str]): Headers with all keys lowercase
 
     Raises:
         AuditPrincipalError: Cannot get a principal from the headers