fix: bump minimum cryptography dependency to >= 39.0.0

[xyaz1313]

Problem

The unsafe_skip_rsa_key_validation kwarg used in jwk.py at lines 842 and 1000 was added to the cryptography library in version 39.0.0 (PR #7667).

However, pyproject.toml only requires cryptography >= 3.4, which means pip can install jwcrypto 1.5.7 with an older cryptography that lacks this parameter, causing:

TypeError: load_pem_private_key() got an unexpected keyword argument "unsafe_skip_rsa_key_validation"

Fix

Bump the minimum version in pyproject.toml:

-    "cryptography >= 3.4",
+    "cryptography >= 39.0.0",

Verification

• Confirmed unsafe_skip_rsa_key_validation is present in both load_pem_private_key() and RSAPrivateNumbers.private_key() starting from cryptography 39.0.0
• The parameter name has not changed since introduction (a rename PR #8470 was closed without merging)
• All existing functionality is preserved — this is purely a dependency constraint fix

Fixes #376

[simo5]

Thanks!