Fix cert_validate timezone warnings

[longzheng]

Was trying the new cert validation method and noticed some deprecation warnings about comparing not_valid_before and not_valid_after. Fixed to use the _utc properties.

C:\Users\longz\Documents\GitHub\SEP2-Tools>python -m sep2tools cert-lfdi cert.pem
The LFDI is: 913D-F605-5ACB-D322-F2C9-82DA-EE0C-0DA2-962C-8F55
C:\Users\longz\Documents\GitHub\SEP2-Tools\sep2tools\cert_validate.py:42: CryptographyDeprecationWarning: Properties that return a naïve datetime object have been deprecated. Please switch to not_valid_before_utc.
  if not cert.not_valid_before <= current_time:
C:\Users\longz\Documents\GitHub\SEP2-Tools\sep2tools\cert_validate.py:47: CryptographyDeprecationWarning: Properties that return a naïve datetime object have been deprecated. Please switch to not_valid_after_utc.
  if not current_time <= cert.not_valid_after:
C:\Users\longz\Documents\GitHub\SEP2-Tools\sep2tools\cert_validate.py:52: CryptographyDeprecationWarning: Properties that return a naïve datetime object have been deprecated. Please switch to not_valid_after_utc.
  if cert.not_valid_after != INDEF_EXPIRY:

[longzheng]

I noticed in cert_create.py you're using a different module from dateutil import tz and tz.UTC.

But I saw from the documentation the datetime module has a timezone export that seems to do the same thing, not sure if I'm missing something since I'm not experienced with Python.

If it is the same thing then potential cert_create.py could be updated to use this and then the dateutil module is no longer needed?

SEP2-Tools/sep2tools/cert_create.py

Line 342 in 5aab6b4

valid_from = datetime.now(tz=tz.UTC)