fix fencepost error when getting source inside decorator in interpreter (fixes #603)

[timkpaine]

Summary

I believe this fixes an off-by-one error for functions defined in the interpreter.

I am not sure how to test this as it only runs when directly instantiated in an interpreter.
Fixes #603.

Inline test:

from dill.source import getsource

def my_decorator(foo):
    s = getsource(foo)
    print(s)
    assert s.endswith("return 1\n")

@my_decorator
def foo():
  return 1

Before

@my_decorator
def foo():

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in my_decorator
AssertionError

After

@my_decorator
def foo():
    return 1
>>>

Please let me know any suggestions for how to implement a test for this!

Checklist

Documentation and Tests

• Added relevant tests that run with python tests/__main__.py, and pass.
• Added relevant documentation that builds in sphinx without error.
• Added new features that are documented with examples.
• Artifacts produced with the main branch work as expected under this PR.

Release Management

• Added "Fixes #NNN" in the PR body, referencing the issue (#NNN) it closes.
• Added a comment to issue #NNN, linking back to this PR.
• Added rationale for any breakage of backwards compatibility.
• Requested a review.

[mmckerns]

thanks for the patch