Check of elapsed time between two archives is approximative

[Kraymer]

======================================================================
FAIL: test_number_of_archives (test_cronicle.Test)
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Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.6.7/lib/python3.6/site-packages/mock/mock.py", line 1305, in patched
    return func(*args, **keywargs)
  File "/home/travis/build/Kraymer/cronicle/test/test_cronicle.py", line 104, in test_number_of_archives
    {"foo_2019-12-01_09h", "foo_2020-01-01_09h", "foo_2020-02-01_09h",},
AssertionError: Items in the first set but not the second:
'foo_2019-12-31_09h'
'foo_2020-01-30_09h'
Items in the second set but not the first:
'foo_2020-02-01_09h'
'foo_2020-01-01_09h'

[Kraymer]

Fixed by 8b32ac3