Implement lazy loading for jwt and cryptography in auth

dlt/sources/helpers/rest_client/auth.py

@@ -1,32 +1,34 @@
 from base64 import b64encode
 import math
-from typing import List, Dict, Final, Literal, Optional, Union, Any, cast, Iterable
+from typing import (
+    List,
+    Dict,
+    Final,
+    Literal,
+    Optional,
+    Union,
+    Any,
+    cast,
+    Iterable,
+    TYPE_CHECKING,
+)
 from dlt.sources.helpers import requests
 from requests.auth import AuthBase
 from requests import PreparedRequest  # noqa: I251
 import pendulum
 
 from dlt.common.exceptions import MissingDependencyException
 
-try:
-    import jwt
-except ModuleNotFoundError:
-    raise MissingDependencyException("dlt OAuth helpers", ["PyJWT"])
-
-try:
-    from cryptography.hazmat.backends import default_backend
-    from cryptography.hazmat.primitives import serialization
-    from cryptography.hazmat.primitives.asymmetric.types import PrivateKeyTypes
-except ModuleNotFoundError:
-    raise MissingDependencyException("dlt OAuth helpers", ["cryptography"])
-
-from dlt import config, secrets
 from dlt.common import logger
 from dlt.common.configuration.specs.base_configuration import configspec
 from dlt.common.configuration.specs import CredentialsConfiguration
 from dlt.common.configuration.specs.exceptions import NativeValueError
 from dlt.common.typing import TSecretStrValue
 
+if TYPE_CHECKING:
+    from cryptography.hazmat.primitives.asymmetric.types import PrivateKeyTypes
+else:
+    PrivateKeyTypes = Any
 
 TApiKeyLocation = Literal[
     "header", "cookie", "query", "param"
@@ -161,6 +163,11 @@ def is_token_expired(self) -> bool:
         return not self.token_expiry or pendulum.now() >= self.token_expiry
 
     def obtain_token(self) -> None:
+        try:
+            import jwt
+        except ModuleNotFoundError:
+            raise MissingDependencyException("dlt OAuth helpers", ["PyJWT"])
+
         payload = self.create_jwt_payload()
         data = {
             "grant_type": "urn:ietf:params:oauth:grant-type:jwt-bearer",
@@ -191,7 +198,13 @@ def create_jwt_payload(self) -> Dict[str, Union[str, int]]:
             "scope": cast(str, self.scopes),
         }
 
-    def load_private_key(self) -> PrivateKeyTypes:
+    def load_private_key(self) -> "PrivateKeyTypes":
+        try:
+            from cryptography.hazmat.backends import default_backend
+            from cryptography.hazmat.primitives import serialization
+        except ModuleNotFoundError:
+            raise MissingDependencyException("dlt OAuth helpers", ["cryptography"])
+
         private_key_bytes = self.private_key.encode("utf-8")
         return serialization.load_pem_private_key(
             private_key_bytes,