Fix import loop issue

The following discussion from !143 (merged) should be addressed:

• @moul started a discussion: (+1 comment)

  Is there anything that can be done for this import loop issue? Did you try to understand the importation process order? I haven't check in details, but this is usually due to a wrong architectural design.

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• Get rid of convert_time(), and generalize pendulum usage
• Remove following weird imports:

grep -rnA1 "had to import" silkaj
silkaj/money.py:27:# had to import from wot to avoid loop dependencies
silkaj/money.py-28-from silkaj.wot import check_public_key
--
silkaj/auth.py:26:# had to import pubkey_with_checksum from wot to avoid loop dependency.
silkaj/auth.py-27-from silkaj.wot import pubkey_with_checksum
--
silkaj/wot.py-29-from silkaj.tui import convert_time, pubkey_with_checksum