CLN: replace %s syntax with .format in pandas.core.reshape
Progress towards #16130

- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Replaced `%s` syntax with `.format` in pandas.core.reshape.  Additionally, made some of the existing positional `.format` code more explicit.

@jschendel : Very nice!  Ping when green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=h1) Report
> Merging [#17252](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0f25426eac5c0097214df78b45125ac039c85770?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `90.9%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17252/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17252      +/-   ##
==========================================
- Coverage   91.03%   91.01%   -0.02%     
==========================================
  Files         162      162              
  Lines       49527    49530       +3     
==========================================
- Hits        45086    45080       -6     
- Misses       4441     4450       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.79% <90.9%> (ø)` | :arrow_up: |
| #single | `40.25% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/tile.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS90aWxlLnB5) | `90.25% <100%> (ø)` | :arrow_up: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `95.36% <100%> (ø)` | :arrow_up: |
| [pandas/core/reshape/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9jb25jYXQucHk=) | `97.6% <75%> (ø)` | :arrow_up: |
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `99.28% <88.88%> (ø)` | :arrow_up: |
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.26% <93.33%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=footer). Last update [0f25426...388c322](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=h1) Report
> Merging [#17252](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/924b43359b2450cd1e6e364c468c30ee7694f0c1?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `91.42%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17252/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17252      +/-   ##
==========================================
+ Coverage      91%   91.01%   +<.01%     
==========================================
  Files         162      162              
  Lines       49531    49535       +4     
==========================================
+ Hits        45078    45085       +7     
+ Misses       4453     4450       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.79% <91.42%> (+0.02%)` | :arrow_up: |
| #single | `40.25% <2.85%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `95.36% <100%> (ø)` | :arrow_up: |
| [pandas/core/reshape/tile.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS90aWxlLnB5) | `90.25% <100%> (ø)` | :arrow_up: |
| [pandas/core/reshape/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9jb25jYXQucHk=) | `97.6% <75%> (ø)` | :arrow_up: |
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `99.28% <90.9%> (ø)` | :arrow_up: |
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.26% <93.33%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.05% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=footer). Last update [924b433...b0672cb](https://codecov.io/gh/pandas-dev/pandas/pull/17252?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @gfyoung : Green!  Had an annoying Python 2 related unicode issue earlier, but was able to solve it. @jschendel : Nice!  Thanks for doing this!