v4.1.0

4.1.0: seller country, sale type and comp currency (#13)

EbayListing gains currency, seller_country and sale_type. Card comps from
ebay.de and ebay.es are priced in EUR, so currency has to be read per row.
CardmarketListing gains seller_country.

New listing filters:
- seller_country and exclude_seller_country on the eBay and Cardmarket
  listings for cards and sealed. They take an array or a comma-separated
  string, and arrays are joined with commas on the wire.
- grade_qualifier on card eBay comps: pristine, black_label or none.
- sale_type on card eBay comps, one value or an array.

ErrorCode adds missing_api_key and invalid_api_key, which the API now sends
on a 401 instead of unauthorized. The README covers the new fields, Spanish
cards, and the updated meaning of an unknown attribution.

Everything is additive, so this is a minor bump.

Verified with npm run typecheck and npm run build.