[internal] mdtest silently ignores "python" code blocks

[JelleZijlstra]

If you use a code block tagged with "python" instead of "py" in an mdtest, it's silently ignored. For example, after this patch the relevant test still passes:

diff --git a/crates/ty_python_semantic/resources/mdtest/import/case_sensitive.md b/crates/ty_python_semantic/resources/mdtest/import/case_sensitive.md
index f822ba47e2..8956e4dac1 100644
--- a/crates/ty_python_semantic/resources/mdtest/import/case_sensitive.md
+++ b/crates/ty_python_semantic/resources/mdtest/import/case_sensitive.md
@@ -22,7 +22,7 @@ class Foo:
 ```python
 from a import Foo
 
-reveal_type(Foo().x)  # revealed: int
+reveal_type(Foo().x)  # revealed: int42
 ```
 

(Noticed this while working on something else; might fix it later but no guarantees.)