Always use `datetime` to generate timestamps, fixes #39

[bibz]

Only rely on datetime to generate timestamps, and not time and datetime.

This makes it easier for downstream projects to freeze time and get reproducible timestamps. freezegun is a nice and easy way to do so, but I stuck to regular mocks for this PR to avoid a new dependency.

With freezegun:

- @pytest.mark.usefixtures("freeze_random", "freeze_datetime")
+ @pytest.mark.usefixtures("freeze_random")
+ @freezegun.freeze_time(datetime.datetime(2021, 10, 2, 20, 17, 35))
  def test_two_debits(sdd, today):
      payment1 = {

[bibz]

This PR fixes issue #39 .

[raphaelm]

Merged manually