Fix typing so that tests running on python 3.8 pass

Author: xvaier

src/flareio/api_client.py

@@ -21,7 +21,7 @@
 
 
 _API_DOMAIN_DEFAULT: str = "api.flare.io"
-_ALLOWED_API_DOMAINS: frozenset[str] = frozenset(
+_ALLOWED_API_DOMAINS: t.FrozenSet[str] = frozenset(
     {
         _API_DOMAIN_DEFAULT,
         "api.eu.flare.io",

src/flareio/auth.py

@@ -1,14 +1,16 @@
 from requests import PreparedRequest
 from requests.auth import AuthBase
 
+import typing as t
+
 
 class _StaticHeadersAuth(AuthBase):
     def __init__(
         self,
         *,
-        headers: dict[str, str],
+        headers: t.Dict[str, str],
     ) -> None:
-        self._headers: dict[str, str] = headers
+        self._headers: t.Dict[str, str] = headers
 
     def __call__(
         self,