BUG: Replace internal use of loc with reindex in DataFrame append
The issue here was that the DataFrame append method was using .loc, which only throws a warning now, but would eventually throw a KeyError whenever that went into effect. Just swapped out that use of .loc for .reindex.

- [x] closes #22252
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=h1) Report
> Merging [#26022](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6c613c8e40c2210826bb2450803ec52658d5667c?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26022/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26022      +/-   ##
==========================================
- Coverage   91.82%   91.81%   -0.01%     
==========================================
  Files         175      175              
  Lines       52551    52551              
==========================================
- Hits        48256    48252       -4     
- Misses       4295     4299       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.38% <100%> (ø)` | :arrow_up: |
| #single | `40.72% <0%> (-0.14%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26022/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <100%> (-0.12%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26022/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26022?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/26022?src=pr&el=footer). Last update [6c613c8...1bce70f](https://codecov.io/gh/pandas-dev/pandas/pull/26022?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/26022?src=pr&el=h1) Report
> Merging [#26022](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6de813364fd4ebfd16e2794e5ac3422d3bef87e7?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26022/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26022      +/-   ##
==========================================
- Coverage   91.82%   91.82%   -0.01%     
==========================================
  Files         175      175              
  Lines       52539    52539              
==========================================
- Hits        48246    48242       -4     
- Misses       4293     4297       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.38% <100%> (ø)` | :arrow_up: |
| #single | `40.73% <0%> (-0.14%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26022/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <100%> (-0.12%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26022/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26022?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/26022?src=pr&el=footer). Last update [6de8133...4944b99](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Sorry about that. In progress. Prior to this fix, the test still would have passed, but a FutureWarning would have been thrown, and eventually a KeyError. Should I also throw in a check that a FutureWarning and/or KeyError is not thrown? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=h1) Report
> Merging [#26022](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/dc86509b44b3fb0cd9a1a6d6ed564b082dc50848?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26022/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26022      +/-   ##
==========================================
- Coverage   91.98%   91.97%   -0.01%     
==========================================
  Files         175      175              
  Lines       52377    52377              
==========================================
- Hits        48180    48176       -4     
- Misses       4197     4201       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.53% <100%> (ø)` | :arrow_up: |
| #single | `40.71% <0%> (-0.15%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26022/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <100%> (-0.12%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26022/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26022?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/26022?src=pr&el=footer). Last update [dc86509...ff96e0c](https://codecov.io/gh/pandas-dev/pandas/pull/26022?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > Should I also throw in a check that a FutureWarning and/or KeyError is not thrown?

The way that the test is written is fine.  Just need to separate it out. Anything else to do here? lgtm. can you merge master; ping on green. All green! thanks @cbertinato 