Price-parser

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* Improved parsing of prices without digits before a decimal point ('.75'),
https://github.com/scrapinghub/price-parser/pull/42
* Fix parsing of price with non-breaking spaces
https://github.com/scrapinghub/price-parser/pull/43

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* Fixed installation issue on some Windows machines.

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* Improved Korean and Japanese currency detection.
* Declare Python 3.8 support.

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* Redundant $ signs are no longer returned as a part of currency, e.g.
for ``SGD$ 100`` currency would be ``SGD``, not ``SGD$``.

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* New ``Price.fromstring`` argument ``decimal_separator`` allows to override
decimal separator for the cases where it is known
(i.e. disable decimal separator detection);
* NTD and RBM unofficial currency names are added;
* quantifiers in regular expressions are made non-greedy, which provides
a small speedup;
* test improvements.

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* Declare price-parser as providing type annotations (pep-561). This enables
better type checking for projects using price-parser.
* improved test coverage