Update isxn.py to account for ISBN standard changes.

As discussed here https://www.isbn-international.org/content/changes-united-states-isbn-prefixes the valid ISBN range is being extended to include ISBN-979-8

Author: d-r-jenkins

barcode/isxn.py

@@ -45,7 +45,7 @@ def __init__(self, isbn, writer=None):
         self.isbn13 = isbn
         if isbn[:3] not in ("978", "979"):
             raise WrongCountryCodeError("ISBN must start with 978 or 979.")
-        if isbn[:3] == "979" and isbn[3:5] not in ("10", "11"):
+        if isbn[:3] == "979" and isbn[3:4] not in ("1", "8"):
             raise BarcodeError("ISBN must start with 97910 or 97911.")
         super().__init__(isbn, writer)