Changing B2C implementation to verify policy format (#114)

Author: abhidnya13

msal/authority.py

@@ -48,7 +48,9 @@ def __init__(self, authority_url, validate_authority=True,
         self.proxies = proxies
         self.timeout = timeout
         authority, self.instance, tenant = canonicalize(authority_url)
-        is_b2c = any(self.instance.endswith("." + d) for d in WELL_KNOWN_B2C_HOSTS)
+        parts = authority.path.split('/')
+        is_b2c = any(self.instance.endswith("." + d) for d in WELL_KNOWN_B2C_HOSTS) or (
+            len(parts) == 3 and parts[2].lower().startswith("b2c_"))
         if (tenant != "adfs" and (not is_b2c) and validate_authority
                 and self.instance not in WELL_KNOWN_AUTHORITY_HOSTS):
             payload = instance_discovery(