MAINT: Reset sys.path if docstrings import fails
Follow-up to https://github.com/pandas-dev/pandas/pull/22413#discussion_r211459766

cc @WillAyd 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22456?src=pr&el=h1) Report
> Merging [#22456](https://codecov.io/gh/pandas-dev/pandas/pull/22456?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/68273a7a8b05049b311ea207f78e6d6f30c6eaf5?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/22456/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22456?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22456   +/-   ##
=======================================
  Coverage   92.05%   92.05%           
=======================================
  Files         169      169           
  Lines       50733    50733           
=======================================
  Hits        46702    46702           
  Misses       4031     4031
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.46% <ø> (ø)` | :arrow_up: |
| #single | `42.24% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22456?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/22456?src=pr&el=footer). Last update [68273a7...c7d0f3c](https://codecov.io/gh/pandas-dev/pandas/pull/22456?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks