gh-41071: Fix missing file error with sage_getdoc
see the newly added test, previously it.
The reason this affect users is that if in a shell you type
```
sage: class C:
....: '''
....: docs
....: '''
sage: C?
```
then you'd get an error. After this change, it prints out something
somewhat sensible. (also added as a test)
I'm not 100% sure what's a good way out on it be this one. On one hand,
you ought to make `sage_getfile` behave the same way as
https://docs.python.org/3/library/inspect.html#inspect.getfile; on the
other hand it is not documented by
https://docs.python.org/3/library/inspect.html#inspect.getfile that it
may raise `OSError`, so IPython isn't prepared to deal with it.
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preview.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/41071
Reported by: user202729
Reviewer(s): Chenxin Zhong
