bump python Markdown package to version 3.3.7

ericseppanen · ericseppanen · commit b341e893910d · 2023-05-11T20:57:31.000-07:00
The version used to build the site was bumped in rust-lang#3338, but the version used in CI was not.
diff --git a/tools/requirements.txt b/tools/requirements.txt
@@ -1,2 +1,2 @@
 beautifulsoup4==4.10.0
-Markdown==3.3.6
+Markdown==3.3.7
