Use not_valid_after_utc instead of the deprecated not_valid_after (#52)

This requires Cryptography >= 42.0.0 and hence Python >= 3.7, which
is not an issue, as requests itself also requires Python >= 3.7.

requests_pkcs12.py

@@ -34,7 +34,7 @@
     from ssl import PROTOCOL_SSLv23 as default_ssl_protocol
 
 def _check_cert_not_after(cert):
-    cert_not_after = cert.not_valid_after
+    cert_not_after = cert.not_valid_after_utc
     if cert_not_after < datetime.datetime.utcnow():
         raise ValueError('Client certificate expired: Not After: {cert_not_after:%Y-%m-%d %H:%M:%SZ}'.format(**locals()))
 

setup.py

@@ -8,9 +8,9 @@
     author='Volker Diels-Grabsch',
     author_email='volker.diels-grabsch@m-click.aero',
     license='ISC',
-    python_requires='>=3.6',
+    python_requires='>=3.7',
     install_requires=[
-        'cryptography (>=3.4.7)',
+        'cryptography (>=42.0.0)',
         'requests (>=2.26.0)',
     ],
     extras_require={