markdown: improve preprocessors type (#4588)

* markdown: improve preprocessors type

https://github.com/Python-Markdown/markdown/blob/b701c34ebd7b2d0eb319517b9a275ddf0c89608d/markdown/preprocessors.py#L46

* Apply suggestions from code review

Author: hauntsaninja

third_party/2and3/markdown/preprocessors.pyi

@@ -1,14 +1,14 @@
-from typing import Any, Pattern
+from typing import Any, Iterable, List, Pattern
 
 from . import util
 
 def build_preprocessors(md, **kwargs): ...
 
 class Preprocessor(util.Processor):
-    def run(self, lines) -> None: ...
+    def run(self, lines: List[str]) -> List[str]: ...
 
 class NormalizeWhitespace(Preprocessor):
-    def run(self, lines): ...
+    def run(self, lines: Iterable[str]) -> List[str]: ...
 
 class HtmlBlockPreprocessor(Preprocessor):
     right_tag_patterns: Any
@@ -17,10 +17,10 @@ class HtmlBlockPreprocessor(Preprocessor):
     attrs_re: Any
     left_tag_re: Any
     markdown_in_raw: bool = ...
-    def run(self, lines): ...
+    def run(self, lines: Iterable[str]) -> List[str]: ...
 
 class ReferencePreprocessor(Preprocessor):
     TITLE: str = ...
     RE: Pattern
     TITLE_RE: Pattern
-    def run(self, lines): ...
+    def run(self, lines: List[str]) -> List[str]: ...