$ ipython

In [1]: 1626111425534 / 1e3
Out[1]: 1626111425.534

In [2]: timestamp = 1626111425534

In [7]:  datetime.fromtimestamp(int(timestamp) / 1e3)
Out[7]: datetime.datetime(2021, 7, 12, 19, 37, 5, 534000)

In [8]: str( datetime.fromtimestamp(int(timestamp) / 1e3) )
Out[8]: '2021-07-12 19:37:05.534000'

In [9]: import iso8601

In [10]: date = datetime.fromtimestamp(timestamp / 1e3)

In [11]: str ( date )
Out[11]: '2021-07-12 19:37:05.534000'

In [12]: iso8601.parse_date(str ( date ) )
Out[12]: datetime.datetime(2021, 7, 12, 19, 37, 5, 534000, tzinfo=datetime.timezone.utc)