mypy: Enable new error explicit-override.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/markdown/include.py

@@ -6,6 +6,7 @@ from xml.etree.ElementTree import Element
 from markdown import Extension, Markdown
 from markdown.blockparser import BlockParser
 from markdown.blockprocessors import BlockProcessor
+from typing_extensions import override
 
 from zerver.lib.exceptions import InvalidMarkdownIncludeStatementError
 from zerver.lib.markdown.priorities import BLOCK_PROCESSOR_PRIORITIES
@@ -16,6 +17,7 @@ class IncludeExtension(Extension):
         super().__init__()
         self.base_path = base_path
 
+    @override
     def extendMarkdown(self, md: Markdown) -> None:
         md.parser.blockprocessors.register(
             IncludeBlockProcessor(md.parser, self.base_path),
@@ -31,6 +33,7 @@ class IncludeBlockProcessor(BlockProcessor):
         super().__init__(parser)
         self.base_path = base_path
 
+    @override
     def test(self, parent: Element, block: str) -> bool:
         return bool(self.RE.search(block))
 
@@ -46,6 +49,7 @@ class IncludeBlockProcessor(BlockProcessor):
 
         return "\n".join(lines)
 
+    @override
     def run(self, parent: Element, blocks: List[str]) -> None:
         self.parser.state.set("include")
         self.parser.parseChunk(parent, self.RE.sub(self.expand_include, blocks.pop(0)))