REF/TST: use pytest builtin monkeypatch fixture and remove mock fixture
- [n/a ] xref https://github.com/pandas-dev/pandas/pull/24557#issuecomment-451174496
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [n/a ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24624?src=pr&el=h1) Report
> Merging [#24624](https://codecov.io/gh/pandas-dev/pandas/pull/24624?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cb31b2b0912c99e40b22a26e598e640548ea8f14?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24624/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24624?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24624      +/-   ##
==========================================
- Coverage   92.38%   92.37%   -0.01%     
==========================================
  Files         166      166              
  Lines       52395    52395              
==========================================
- Hits        48403    48402       -1     
- Misses       3992     3993       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24624?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24624/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24624?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/24624?src=pr&el=footer). Last update [cb31b2b...9460d52](https://codecov.io/gh/pandas-dev/pandas/pull/24624?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @simonjayhawkins in followup could add a check for ``mock`` in the ``ci/code_checks`` (meaning we should *not* find it) thanks!