Click https://click.palletsprojects.com/en/7.x/changelog/#version-7-1-2
 - Fix test: the error message changed from double to simple cotes
PyNaCl: https://github.com/pyca/pynacl/blob/master/CHANGELOG.rst#140-2020-05-25
Pytest: https://docs.pytest.org/en/latest/changelog.html#pytest-5-4-3-2020-06-02

* transaction_amount() now returns the amounts list.

* modify tests in tests_tx.py for transaction_amount to match new behavior
* create unit tests in tests_unit_tx.py

* both constants imported to tx.py
* MINIMAL_TX_AMOUNT used in --amount click option
* import silkaj.money and change related functions calls

* introduce one test case for send_transaction()

* Breaking change: Rename `--output` option to `--recipient`
* Add extra small options to ease passing multiple amounts and recipients:
  * `-a/--amount`
  * `-d/--amountUD`
  * `-r/--recipient`
* Add possibility to pass multiple options:
  * Breaking change: recipients public keys are no longer `:` separated: `-r A -r B`
* Store the options in variables names in plural for preciseness
* Fix: Handle the case when no option specifying the amount is passed
* Restrict `--allSources` option to a single recipient, case not handle for now
* Extend tests with two upper checks
* Improve help