Fix deprecation warnings: .format() -> .format_string(), .assertEqual…

[SeanMcAuliffe]

python -m pytest produced deprecation warnings concerning the use of the .format() and .assertEquals() methods. This PR replaces them with the suggested upgrades.

The results of the test suite after this change are below.

============================================================================================== test session starts ==============================================================================================
platform linux -- Python 3.9.2, pytest-7.1.1, pluggy-1.0.0
rootdir: /home/seanm/github/InvoiceGenerator
collected 42 items

tests/test_api.py ..................................                                                                                                                                                      [ 80%]
tests/test_generator.py ..                                                                                                                                                                                [ 85%]
tests/test_pdf.py ....                                                                                                                                                                                    [ 95%]
tests/test_pohoda.py ..                                                                                                                                                                                   [100%]

=============================================================================================== warnings summary ================================================================================================
InvoiceGenerator/conf.py:13: 1 warning
tests/test_generator.py: 12 warnings
tests/test_pdf.py: 100 warnings
  /home/seanm/github/InvoiceGenerator/InvoiceGenerator/conf.py:13: DeprecationWarning: parameter codeset is deprecated
    t = gettext.translation(

tests/test_generator.py::TestGenerator::test_gen
tests/test_pdf.py::TestBaseInvoice::test_generate
tests/test_pdf.py::TestBaseInvoice::test_generate
tests/test_pdf.py::TestBaseInvoice::test_generate_proforma
tests/test_pdf.py::TestBaseInvoice::test_generate_with_vat
  /home/seanm/github/InvoiceGenerator/InvoiceGenerator/pdf.py:105: DeprecationWarning: currency attribute is deprecated, use currency_locale instead
    warnings.warn("currency attribute is deprecated, use currency_locale instead", DeprecationWarning)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================================================================================= 42 passed, 118 warnings in 0.51s ========================================================================================```