ENH: use TypeVar on cached.doit() signature

src/ampform/sympy/cached.py

@@ -8,25 +8,28 @@
 
 if TYPE_CHECKING:
     from collections.abc import Mapping
+    from typing import TypeVar
 
     import sympy as sp
 
+    SympyObject = TypeVar("SympyObject", bound=sp.Basic)
+
 
 @cache_to_disk
-def doit(unevaluated_expr: sp.Expr) -> sp.Expr:
+def doit(expr: SympyObject) -> SympyObject:
     """Perform :meth:`~sympy.core.basic.Basic.doit` and cache the result to disk.
 
     The cached result is fetched from disk if the hash of the original expression is the
     same as the hash embedded in the filename (see :func:`.get_readable_hash`).
 
     Args:
-        unevaluated_expr: A `sympy.Expr <sympy.core.expr.Expr>` on which to call
+        expr: A `sympy.Expr <sympy.core.expr.Expr>` on which to call
             :meth:`~sympy.core.basic.Basic.doit`.
 
     .. versionadded:: 0.14.4
     .. automodule:: ampform.sympy._cache
     """
-    return unevaluated_expr.doit()
+    return expr.doit()
 
 
 @cache_to_disk