Maintenance: make time conversion script platform-independent

[SidestreamColdMelon]

This PR replaces time.sh script with the equivalent time.py script. The functionality and the interface of the new script stays exactly the same, but now it can be run across platforms. In comparison, the previous script didn't work on macOS and after a trial to fix it (see d623861) I decided to rewrite it in python to avoid inconsistencies.

[amusingaxl]

The scripts are not equivalent.

With the bash version, if I run:

make time date="2023-12-01T00:00:00Z"

I get:

Fri Dec  1 12:00:00 AM UTC 2023
1701388800

However, the python script does not work with the same arguments:

Traceback (most recent call last):
  File "/home/anon/work/makerdao/spells-mainnet/./scripts/time.py", line 20, in <module>
    utc_date = datetime.strptime(date, DATE_FORMAT).replace(tzinfo=timezone.utc)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/_strptime.py", line 568, in _strptime_datetime
    tt, fraction, gmtoff_fraction = _strptime(data_string, format)
                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/_strptime.py", line 349, in _strptime
    raise ValueError("time data %r does not match format %r" %
ValueError: time data '2023-12-01T00:00:00Z' does not match format '%Y-%m-%d %H:%M:%S'
make: *** [Makefile:21: time] Error 1

We should probably be less strict about date parsing. At least we should allow ISO8601 strings like the one above.

[amusingaxl]

Looks like there's a good way to parse ISO8601 dates in Python 3:
https://stackoverflow.com/a/49784038/1798341

[SidestreamColdMelon]

That was actually my undocumented intention to make script less smart and more strict with parsing. But supporting a single standard like ISO8601 is also a good choice. I've updated the script!

[amusingaxl]

LGTM.
Should we also port this to Goerli?
It will be deprecated soon, and we don't have many uses for it there, but it's fairly a small change.
https://github.com/makerdao/spells-goerli/blob/master/scripts/time.sh