bpo-28249: fix lineno location for empty DocTest instances

[sobolevn]

This patch fixes lineno for DocTest instances for objects that don't have __doc__ attribute.

In the original issue this was used as an example:

def a():
    # """A"""
    pass

def b():  # line 5
    """B"""
    pass

def c():
    # """C"""
    pass

Before this patch, doctest.DocTestFinder(exclude_empty=False).find(example_module) was showing:

[
  <DocTest example from example.py:5 (no examples)>,
  <DocTest example.a from example.py:5 (no examples)>,
  <DocTest example.b from example.py:5 (no examples)>,
  <DocTest example.c from example.py:None (no examples)>,
]

Notice that lineno was 5 for example and example.a. Which is clearly wrong, because example.b is on the 5th line. But, example.c had lineno=None.

DocTest docs clearly state:

• lineno: The line number within filename where this DocTest
  begins, or None if the line number is unavailable. This
  line number is zero-based, with respect to the beginning of
  the file.

So, now lineno is set to None when __doc__ cannot be found. Now, the result for our example module will be:

[
  <DocTest example from example.py:None (no examples)>, 
  <DocTest example.a from example.py:None (no examples)>,
  <DocTest example.b from example.py:5 (no examples)>, 
  <DocTest example.c from example.py:None (no examples)>, 
]

Now, everything looks correct.

https://bugs.python.org/issue28249

CC @corona10 as my mentor 🙂

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[MaxwellDupre]

Ran 58 tests in 4.543s
OK
Looks ok.

[ambv]

I rebased and resolved a conflict with main.

[miss-islington]

Thanks @sobolevn for the PR, and @ambv for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9, 3.10, 3.11.
🐍🍒⛏🤖

[miss-islington]

Sorry, @sobolevn and @ambv, I could not cleanly backport this to 3.10 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 8db2b3b6878aba9f12844526bce966b7eed81aee 3.10

[miss-islington]

Sorry @sobolevn and @ambv, I had trouble checking out the 3.9 backport branch.
Please backport using cherry_picker on command line.
cherry_picker 8db2b3b6878aba9f12844526bce966b7eed81aee 3.9

[bedevere-bot]

GH-92978 is a backport of this pull request to the 3.11 branch.

[bedevere-bot]

GH-92981 is a backport of this pull request to the 3.10 branch.