[MRG] f-string updates for issue #29547
Addresses, in part, https://github.com/pandas-dev/pandas/issues/29547 

…ape/pivot, reshape/reshape

- [ x] xref #29547 
- [n/a ] tests added / passed
- [x ] passes `black pandas`
- [ x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [n/a ] whatsnew entry

Hello @abbiepopa! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:













There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-02-03 04:07:53 UTC Thank you @WillAyd , I covered the changes you requested. I'm having some trouble figuring out why I'm failing the current tests. LMK if it's obvious to you, otherwise I will continue investigating. Thanks, Abbie!

> I'm having some trouble figuring out why I'm failing the current tests

If you click on 'details' next to, say, "CI / Checks", you'll see an 'X' near 'linting'. If you expand it, you'll see
```
black --version
black, version 19.10b0
Checking black formatting
would reformat /home/runner/work/pandas/pandas/pandas/core/reshape/concat.py
would reformat /home/runner/work/pandas/pandas/pandas/core/reshape/melt.py
would reformat /home/runner/work/pandas/pandas/pandas/core/reshape/pivot.py
would reformat /home/runner/work/pandas/pandas/pandas/core/reshape/merge.py
Oh no! 💥 💔 💥
4 files would be reformatted, 923 files would be left unchanged.
```
So for a start, you need to run `black` again any file you've changed. For example,

```
black pandas/core/reshape/concat.py
```

The [contributing guide](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#contributing-to-the-code-base) has some more info about this. Thank you, @MarcoGorelli ! Looks like the latest failure is a timeout. ~Is there a way to retrigger the test that timed out?~ Never mind, I found some instructions in the contributor wiki, retrying now. Yay! Thanks @WillAyd and @MarcoGorelli . LMK if you need additional changes etc. Thanks @jreback and @MomIsBestFriend ! I updated based on your reviews. Great thanks @abbiepopa 