refactor: Simplify expression

Sourcery suggested:

    suggestion: Using re.match for a simple lowercase check may be
    overkill.

    Consider replacing the regex with: if not (stage_name.islower() and
    stage_name.isalpha()): ... for better readability and efficiency.

This also removes the dependency on the `re` module.

Author: jmgate

staged_script/staged_script.py

@@ -14,7 +14,6 @@
 from __future__ import annotations
 
 import functools
-import re
 import shlex
 import subprocess
 from argparse import (
@@ -58,7 +57,7 @@ def validate_stage_name(stage_name: str) -> None:
     Raises:
         ValueError:  If the stage name is invalid.
     """
-    if not re.match("^[a-z]+$", stage_name):
+    if not (stage_name.islower() and stage_name.isalpha()):
         message = (
             f"Stage name {stage_name!r} must contain only lowercase letters."
         )