fix: Only replace CSS class in first *highlighting* span

When `pymdownx.highlight` sets `auto_title: true`, another span is rendered before the highlighted code. If we replace its CSS class, we mess its final display. Our solution is not very robust: we only look for spans with CSS classes of 1 or 2 characters instead of 1 or more characters.

Issue-281: #281

Author: pawamoy

src/mkdocstrings_handlers/python/_internal/rendering.py

@@ -190,7 +190,7 @@ def do_format_signature(
     # To fix this, we replace the CSS class in the first span with `nf`,
     # unless we already found an `nf` span.
     if not re.search(r'<span class="nf">', signature):
-        signature = re.sub(r'<span class="[a-z]+">', '<span class="nf">', signature, count=1)
+        signature = re.sub(r'<span class="[a-z]{1,2}">', '<span class="nf">', signature, count=1)
 
     if stash := env.filters["stash_crossref"].stash:
         for key, value in stash.items():