Specify timezone when doing conversion in seen module

[timwoj]

Description

I'm not sure why but the seen command started throwing this exception recently on my installation:

Traceback (most recent call last):
  File "/home/tim/sopel/lib/python3.10/site-packages/sopel/bot.py", line 674, in call_rule
    rule.execute(sopel, trigger)
  File "/home/tim/sopel/lib/python3.10/site-packages/sopel/plugins/rules.py", line 1079, in execute
    exit_code = self._handler(bot, trigger)
  File "/home/tim/sopel/lib/python3.10/site-packages/sopel/modules/seen.py", line 44, in seen
    delta = seconds_to_human((trigger.time - saw).total_seconds())
TypeError: can't subtract offset-naive and offset-aware datetimes

This PR fixes that by swapping from utcfromtimestamp to fromtimestamp and passing in the UTC timezone. The python docs actually recommend using fromtimestamp anyways if needing an offset-aware date time.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

I'm getting failures from make test but nothing within the code that I modified.

[dgw]

This is obsoleted by #2265, yes?

[Exirel]

It is, yes.

[dgw]

@timwoj Thanks for the contribution, and for runtime-testing our unstable branch! Going to close this in favor of the previously approved patch, due to land Soon™ after I sort out this git.io thing GitHub sprung on us this week.