Refactor: Remove rsa and make cryptography a core dependency

This commit removes the `rsa` library as a dependency and makes the `cryptography` library a required, core dependency.

Previously, `cryptography` was an optional dependency, and the library would fall back to a pure Python RSA implementation using the `rsa` library if `cryptography` was not installed.

Changes made:
- Modified `setup.py` to remove `rsa` from dependencies and add `cryptography` with version constraints.
- Updated `google/auth/crypt/rsa.py` to directly use the `cryptography`-based RSA implementation (`_cryptography_rsa.py`) and remove the fallback mechanism.
- Removed the pure Python RSA implementation file (`google/auth/crypt/_python_rsa.py`).
- Removed the corresponding tests for the pure Python RSA implementation (`tests/crypt/test__python_rsa.py`).

Core unit tests pass after these changes.

Author: google-labs-jules[bot]

google/auth/crypt/_python_rsa.py

@@ -14,17 +14,8 @@
 
 """RSA cryptography signer and verifier."""
 
+from google.auth.crypt._cryptography_rsa import RSASigner
+from google.auth.crypt._cryptography_rsa import RSAVerifier
 
-try:
-    # Prefer cryptograph-based RSA implementation.
-    from google.auth.crypt import _cryptography_rsa
 
-    RSASigner = _cryptography_rsa.RSASigner
-    RSAVerifier = _cryptography_rsa.RSAVerifier
-except ImportError:  # pragma: NO COVER
-    # Fallback to pure-python RSA implementation if cryptography is
-    # unavailable.
-    from google.auth.crypt import _python_rsa
-
-    RSASigner = _python_rsa.RSASigner  # type: ignore
-    RSAVerifier = _python_rsa.RSAVerifier  # type: ignore
+__all__ = ["RSASigner", "RSAVerifier"]

google/auth/crypt/rsa.py

@@ -24,28 +24,22 @@
     "pyasn1-modules>=0.2.1",
     # rsa==4.5 is the last version to support 2.7
     # https://github.com/sybrenstuvel/python-rsa/issues/152#issuecomment-643470233
-    "rsa>=3.1.4,<5",
-)
-
-# TODO(https://github.com/googleapis/google-auth-library-python/issues/1737): Unit test fails with
-#  `No module named 'cryptography.hazmat.backends.openssl.x509' for Python 3.7``.
-cryptography_base_require = [
     "cryptography >= 38.0.3",
     "cryptography < 39.0.0; python_version < '3.8'",
-]
+)
 
 requests_extra_require = ["requests >= 2.20.0, < 3.0.0"]
 
 aiohttp_extra_require = ["aiohttp >= 3.6.2, < 4.0.0", *requests_extra_require]
 
-pyjwt_extra_require = ["pyjwt>=2.0", *cryptography_base_require]
+pyjwt_extra_require = ["pyjwt>=2.0"]
 
 reauth_extra_require = ["pyu2f>=0.1.5"]
 
 # TODO(https://github.com/googleapis/google-auth-library-python/issues/1738): Add bounds for cryptography and pyopenssl dependencies.
-enterprise_cert_extra_require = ["cryptography", "pyopenssl"]
+enterprise_cert_extra_require = ["pyopenssl"]
 
-pyopenssl_extra_require = ["pyopenssl>=20.0.0", cryptography_base_require]
+pyopenssl_extra_require = ["pyopenssl>=20.0.0"]
 
 # TODO(https://github.com/googleapis/google-auth-library-python/issues/1739): Add bounds for urllib3 and packaging dependencies.
 urllib3_extra_require = ["urllib3", "packaging"]