Handle forever certificates

[ghost]

Creating a certificate using ssh-keygen with no expiry will cause the parser to throw an OverFlow error:

Traceback (most recent call last):
  File "/tmp/test.py", line 4, in <module>
    cert = SSHCertificate.from_file('/tmp/cert')
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/ssh_certificate_parser/__init__.py", line 102, in from_file
    return cls.from_file(f)
           ^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/ssh_certificate_parser/__init__.py", line 103, in from_file
    return cls.from_bytes(path_or_file_object.read())
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/ssh_certificate_parser/__init__.py", line 155, in from_bytes
    valid_before = datetime.datetime.utcfromtimestamp(valid_before)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
OverflowError: timestamp out of range for platform time_t

If creating a certificate that is valid forever, the valid_after value will be set to 0 and the valid_before value will be set to a high number. Since CPython can only handle up to the year 9999 (ref) any values higher than that should be changed to the year 9999.

[Justintime50]

Looks awesome, thank you! 😃