CLN: replace %s syntax with .format in tslibs/timedeltas
- [X] Progress towards #16130
- [X] tests passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
FWIW I didn't see what the point of the 'even_day' argument was to _repr_base in timedeltas.pyx. From both the test cases provided and the control flow, unless I'm missing something that argument is no different from passing nothing or None explicitly. 

Left alone for now but if you want me to remove it won't be a problem lgtm. IIRC ``even_day``  was copied from Timestamp originally (when I wrote this repr). if its not used, you can remove. I’ll go ahead and update the docstring to remove. Did you want to keep the test cases for backwards compatibility or would you rather I delete and just stick with the cases for None? > I’ll go ahead and update the docstring to remove. Did you want to keep the test cases for backwards compatibility or would you rather I delete and just stick with the cases for None?

ok to delete; these are only tests, yes? It looks like the tests and one piece of code in pandas/io/formats/format.py at line 2277 to change 

    format = ‘even_day’

To

    format = None

It looks like there’s already a test case called test_timedelta in the tests/io/formats/test_format.py module to regression test that. 

Sorry for any formatting issues - sent from my phone.  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18405?src=pr&el=h1) Report
> Merging [#18405](https://codecov.io/gh/pandas-dev/pandas/pull/18405?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c4a2cd362afdfbc21cb62a8e09758226e86735c4?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18405/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18405?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18405      +/-   ##
==========================================
- Coverage   91.36%   91.34%   -0.02%     
==========================================
  Files         164      164              
  Lines       49733    49733              
==========================================
- Hits        45439    45430       -9     
- Misses       4294     4303       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.14% <100%> (ø)` | :arrow_up: |
| #single | `39.62% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18405?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/18405?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `96.01% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18405?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18405?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18405?src=pr&el=footer). Last update [c4a2cd3...03e4d41](https://codecov.io/gh/pandas-dev/pandas/pull/18405?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
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