test_x509: Add type annotations

The file now passes `mypy --strict`.

Author: Synss

src/mbedtls/x509.pyi

@@ -8,7 +8,7 @@ import datetime as dt
 import enum
 import sys
 from pathlib import Path
-from typing import NamedTuple, Sequence, Tuple, Union
+from typing import NamedTuple, Sequence, Tuple, Type, TypeVar, Union
 
 from mbedtls.hashlib import Hash as _Hash
 from mbedtls.pk import ECC, RSA
@@ -39,18 +39,22 @@ class KeyUsage(enum.IntEnum):
 def PEM_to_DER(pem: _PEM) -> _DER: ...
 def DER_to_PEM(der: _DER, text: str) -> _PEM: ...
 
+TCertificate = TypeVar("TCertificate", bound=Certificate)
+
 class Certificate(abc.ABC):
     @classmethod
-    def from_buffer(cls, buffer: bytes) -> Certificate: ...
+    def from_buffer(
+        cls: Type[TCertificate], buffer: bytes
+    ) -> TCertificate: ...
     @classmethod
     @abc.abstractmethod
-    def from_file(cls, path: _Path) -> Certificate: ...
+    def from_file(cls: Type[TCertificate], path: _Path) -> TCertificate: ...
     @classmethod
     @abc.abstractmethod
-    def from_DER(cls, der: _DER) -> Certificate: ...
+    def from_DER(cls: Type[TCertificate], der: _DER) -> TCertificate: ...
     @classmethod
     @abc.abstractmethod
-    def from_PEM(cls, pem: _PEM) -> Certificate: ...
+    def from_PEM(cls: Type[TCertificate], pem: _PEM) -> TCertificate: ...
     def __hash__(self) -> int: ...
     def __eq__(self, other: object) -> bool: ...
     @abc.abstractmethod
@@ -90,7 +94,7 @@ class CRT(Certificate):
     @property
     def subject(self) -> str: ...
     @property
-    def subject_public_key(self) -> object: ...
+    def subject_public_key(self) -> _PKEY: ...
     @property
     def key_usage(self) -> object: ...
     @property