Release 0.4.0

Updated IBAN registry file to release 88 (Sept. 2020).
Updated ISBN ranges file.
Updated GS1 company prefix ranges.
Updated MIC registry to version NOV2020.
Revised checks for Belgian VAT ids.
Renamed 'ISIN.calc_check_digits' to 'ISIN.calc_check_digit'.
Removed Python 2.x compatibility.
Added Python 3.9 support.
Added type hints.
Revised project directory layout.