fix(uri): remove "mailto:" prefix manually

* the use of `lstrip()` here was a bit too aggressive
* `removeprefix()` is only available with python 3.9+

Author: max-moser

src/validators/uri.py

@@ -58,7 +58,7 @@ def uri(value: str, /):
 
     # email
     if value.startswith("mailto:"):
-        return email(value.lstrip("mailto:"))
+        return email(value[len("mailto:"):])
 
     # file
     if value.startswith("file:"):