remove support for {python} and {pip} in commands

cibuildwheel/__main__.py

@@ -417,10 +417,10 @@ def detect_warnings(*, options: Options, identifiers: Iterable[str]) -> list[str
         if any(o and ("{python}" in o or "{pip}" in o) for o in option_values):
             # Reminder: in an f-string, double braces means literal single brace
             msg = (
-                f"{option_name}: '{{python}}' and '{{pip}}' are no longer needed, "
-                "and will be removed in cibuildwheel 3. Simply use 'python' or 'pip' instead."
+                f"{option_name}: '{{python}}' and '{{pip}}' are no longer supported "
+                "and have been removed in cibuildwheel 3. Simply use 'python' or 'pip' instead."
             )
-            warnings.append(msg)
+            raise errors.ConfigurationError(msg)
 
     return warnings
 

cibuildwheel/util.py

@@ -219,12 +219,12 @@ def format_safe(template: str, **kwargs: str | os.PathLike[str]) -> str:
 
 def prepare_command(command: str, **kwargs: PathOrStr) -> str:
     """
-    Preprocesses a command by expanding variables like {python}.
+    Preprocesses a command by expanding variables like {project}.
 
     For example, used in the test_command option to specify the path to the
     project's root. Unmatched syntax will mostly be allowed through.
     """
-    return format_safe(command, python="python", pip="pip", **kwargs)
+    return format_safe(command, **kwargs)
 
 
 def get_build_verbosity_extra_flags(level: int) -> list[str]: