[Key Vault] Fix key instantiation errors. (#41205)

The latest version of `cryptography` now defines `__copy__` as an
abstract method in both RSAPublicKey and RSAPrivateKey.
`KeyVaultRSAPublicKey` and `KeyVaultRSAPrivateKey` need to implement
this method so that they can be successfully instantiated.

This change adds the `__copy__` method to both classes, returning `self`
in each case. This mirrors behavior in `cryptography` and is appropriate
because the key objects are immutable.

Signed-off-by: Paul Van Eck <paulvaneck@microsoft.com>

Author: pvaneck

sdk/keyvault/azure-keyvault-keys/CHANGELOG.md

@@ -7,6 +7,7 @@
 ### Breaking Changes
 
 ### Bugs Fixed
+- Fixed a bug where `KeyVaultRSAPublicKey` and `KeyVaultRSAPrivateKey` were not correctly implementing the `cryptography` library's `RSAPublicKey` and `RSAPrivateKey` interfaces, causing instantiation errors. ([#41205](https://github.com/Azure/azure-sdk-for-python/pull/41205))
 
 ### Other Changes
 
@@ -151,7 +152,7 @@
 ## 4.6.1 (2022-08-11)
 
 ### Other Changes
-- Documentation improvements 
+- Documentation improvements
   ([#25039](https://github.com/Azure/azure-sdk-for-python/issues/25039))
 
 ## 4.6.0b1 (2022-06-07)

sdk/keyvault/azure-keyvault-keys/azure/keyvault/keys/crypto/_models.py

@@ -2,6 +2,7 @@
 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT License.
 # ------------------------------------
+from __future__ import annotations
 from typing import Any, cast, Optional, NoReturn, Union, TYPE_CHECKING
 
 from cryptography.exceptions import InvalidSignature
@@ -308,6 +309,14 @@ def __eq__(self, other: object) -> bool:
             return all(getattr(self._key, field) == getattr(other, field) for field in self._key._FIELDS)
         return False
 
+    def __copy__(self) -> KeyVaultRSAPublicKey:
+        """Returns this instance since it is treated as immutable.
+
+        :returns: This instance.
+        :rtype: ~azure.keyvault.keys.crypto.KeyVaultRSAPublicKey
+        """
+        return self
+
     def verifier(  # pylint:disable=docstring-missing-param,docstring-missing-return,docstring-missing-rtype
         self, signature: bytes, padding: AsymmetricPadding, algorithm: HashAlgorithm
     ) -> NoReturn:
@@ -482,6 +491,14 @@ def signer(  # pylint:disable=docstring-missing-param,docstring-missing-return,d
         """Not implemented. This method was deprecated in `cryptography` 2.0 and removed in 37.0.0."""
         raise NotImplementedError()
 
+    def __copy__(self) -> KeyVaultRSAPrivateKey:
+        """Returns this instance since it is treated as immutable.
+
+        :returns: This instance.
+        :rtype: ~azure.keyvault.keys.crypto.KeyVaultRSAPrivateKey
+        """
+        return self
+
 
 class DecryptResult:
     """The result of a decrypt operation.