Use correct import path in abbr.py

This is the module: https://github.com/Python-Markdown/markdown/blob/master/markdown/blockparser.py

markdown/extensions/abbr.py

@@ -33,7 +33,7 @@
 
 if TYPE_CHECKING:  # pragma: no cover
     from .. import Markdown
-    from ..blockparsers import BlockParser
+    from ..blockparser import BlockParser
 
 
 class AbbrExtension(Extension):