Update required httpx version for v0.18

Author: aiudirog

httpx_gssapi/exceptions.py

@@ -5,10 +5,10 @@
 This module contains the set of exceptions.
 
 """
-from httpx import HTTPError, Request, Response
+from httpx import RequestError, Request, Response
 
 
-class MutualAuthenticationError(HTTPError):
+class MutualAuthenticationError(RequestError):
     """Mutual Authentication Error"""
 
     def __init__(self, *,
@@ -21,5 +21,5 @@ def __init__(self, *,
         )
 
 
-class SPNEGOExchangeError(HTTPError):
+class SPNEGOExchangeError(RequestError):
     """SPNEGO Exchange Failed Error"""

requirements.txt

@@ -1,4 +1,4 @@
-httpx>=0.16,<0.18
+httpx>=0.16,<0.19
 gssapi
 pytest
 k5test