Fix depracted utcfromtimestamp usage

[erkki]

Deprecated as per https://docs.python.org/3/library/datetime.html#datetime.datetime.utcfromtimestamp Fixes: TypeError: can't subtract offset-naive and offset-aware datetimes

Change Summary

Fix depracted utcfromtimestamp usage in dhcp.py.

Deprecated as per https://docs.python.org/3/library/datetime.html#datetime.datetime.utcfromtimestamp

Fixes the following issue:

show dhcp server leases 
Traceback (most recent call last):
  File "/usr/libexec/vyos/op_mode/dhcp.py", line 545, in <module>
    res = vyos.opmode.run(sys.modules[__name__])
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/vyos/opmode.py", line 312, in run
    res = func(**args)
          ^^^^^^^^^^^^
  File "/usr/libexec/vyos/op_mode/dhcp.py", line 335, in _wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/libexec/vyos/op_mode/dhcp.py", line 387, in show_server_leases
    lease_data = _get_raw_server_leases(family=family, pool=pool, sorted=sorted, state=state, origin=origin)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/libexec/vyos/op_mode/dhcp.py", line 135, in _get_raw_server_leases
    data_lease['remaining'] = lease['expire_timestamp'] - datetime.now(timezone.utc)
                              ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
TypeError: can't subtract offset-naive and offset-aware datetimes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

Related PR(s)

Component(s) name

dhcp

Proposed changes

How to test

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Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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• CLI Smoketests (interfaces only) ❌ failed
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• RAID1 tests 👍 passed
• TPM tests 👍 passed

[dmbaturin]

Is there anything you plan to do before marking it ready for review? Seems sensible to me.

[erkki]

Is there anything you plan to do before marking it ready for review? Seems sensible to me.

Not really, just wanted to make sure it fits the contribution guidelines, passes tests etc. I will mark it as ready.