bpo-46306: simplify `CodeType` attribute access in `doctest.py` (GH-30481)

Assume co_firstlineno always exists on types.CodeType objects.

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>

Lib/doctest.py

@@ -1113,7 +1113,7 @@ class DocTestFinder:
         if inspect.istraceback(obj): obj = obj.tb_frame
         if inspect.isframe(obj): obj = obj.f_code
         if inspect.iscode(obj):
-            lineno = getattr(obj, 'co_firstlineno', None)-1
+            lineno = obj.co_firstlineno - 1
 
         # Find the line number where the docstring starts.  Assume
         # that it's the first line that begins with a quote mark.

Misc/NEWS.d/next/Library/2022-01-08-13-53-25.bpo-46306.1_es8z.rst

@@ -0,0 +1,2 @@
+Assume that :class:`types.CodeType` always has :attr:`types.CodeType.co_firstlineno` in
+:mod:`doctest`.