Change span_range typehint to allow Arrow

[laundmo]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• [ ] 🧪 Added tests for changed code.
• [x] 🛠️ All tests pass when run locally (run tox or make test to find out!).
• [x] 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• [x] 📚 Updated documentation for changed code.
• [x] ⏩ Code is up-to-date with the master branch.

No tests were added as no behaviour was changed (only typehints)

Description of Changes

Changes the type hints of Arrow.span_range to correctly indicate it can also be called on Arrow objects.

To pass mypy, the type hints of fromdatetime also had to be changed to indicate it also works with Arrow objects.

Closes: #987

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (522a65b) 100.00% compared to head (a13fa68) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1149 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 10 10 Lines 2326 2326 Branches 400 400 ========================================= Hits 2326 2326 ``` | [Files](https://app.codecov.io/gh/arrow-py/arrow/pull/1149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arrow-py) | Coverage Δ | | |---|---|---| | [arrow/arrow.py](https://app.codecov.io/gh/arrow-py/arrow/pull/1149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arrow-py#diff-YXJyb3cvYXJyb3cucHk=) | `100.00% <100.00%> (ø)` | |

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