Fix ASV import error
#22886 removed `from pandas import Panel` (a left-over from waiting to deprecating `WidePanel`, I believe) from `asv_bench/benchmarks/pandas_vb_common.py` due to linting, and now several benchmarks are failing because they try

```
from .pandas_vb_common import setup, Panel  # noqa
```

This fixes the imports, by adding `Panel` to the other pandas-import appropriately.

The larger question is if the ASV code should be part of the CI somehow to catch such errors. There's a way to execute them in minimal form with `asv dev` (corresponding to `asv run --python=same --quick --show-stderr --dry-run`), see https://asv.readthedocs.io/en/stable/writing_benchmarks.html#running-benchmarks-during-development
Hello @h-vetinari! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`asv_bench/benchmarks/indexing.py`](https://github.com/h-vetinari/pandas/blob/41e5f2998ee54d20a5846d6168c15a674d42db8a/asv_bench/benchmarks/indexing.py) !

 - There are no PEP8 issues in the file [`asv_bench/benchmarks/join_merge.py`](https://github.com/h-vetinari/pandas/blob/41e5f2998ee54d20a5846d6168c15a674d42db8a/asv_bench/benchmarks/join_merge.py) !

 - There are no PEP8 issues in the file [`asv_bench/benchmarks/panel_ctor.py`](https://github.com/h-vetinari/pandas/blob/41e5f2998ee54d20a5846d6168c15a674d42db8a/asv_bench/benchmarks/panel_ctor.py) !

 - There are no PEP8 issues in the file [`asv_bench/benchmarks/panel_methods.py`](https://github.com/h-vetinari/pandas/blob/41e5f2998ee54d20a5846d6168c15a674d42db8a/asv_bench/benchmarks/panel_methods.py) !

 We do have a ci script to run the ASV benchmarks (I don't remember where/if its run off the top of my head)

https://github.com/pandas-dev/pandas-ci/blob/master/ci/asv.sh

IIRC the reason why its not part of the usual CI is because `asv dev` still takes a lot time to run (e.g. was timing out on Travis) Theoretically though once the script is ran, it would warn about failing benchmarks. They're run under https://travis-ci.org/pandas-dev/pandas-ci/

But apparently https://github.com/pandas-dev/pandas/pull/21456/files disabled them, I don't recall why, though if anyone want to investigate that and https://github.com/pandas-dev/pandas/issues/22924 it would be appreciated. Otherwise I'll take a look later in the week. they were timing out 

maybe run these on azure? Thanks. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22978?src=pr&el=h1) Report
> Merging [#22978](https://codecov.io/gh/pandas-dev/pandas/pull/22978?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ee808033bd5c546e7439a06d2ed37b57c9e66844?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22978/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22978?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22978   +/-   ##
=======================================
  Coverage   92.18%   92.18%           
=======================================
  Files         169      169           
  Lines       50823    50823           
=======================================
  Hits        46853    46853           
  Misses       3970     3970
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <ø> (ø)` | :arrow_up: |
| #single | `42.36% <ø> (ø)` | :arrow_up: |


------

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