Update dependency pluggy to v0.8.0 (#2815)

This PR contains the following updates:

| Package | Update | Change | References |
|---|---|---|---|
| pluggy | minor | `==0.7.1` -> `==0.8.0` | [source](https://renovatebot.com/gh/pytest-dev/pluggy) |

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### Release Notes

<details>
<summary>pytest-dev/pluggy</summary>

### [`v0.8.0`](https://renovatebot.com/gh/pytest-dev/pluggy/blob/master/CHANGELOG.rst#pluggy-080-2018-10-15)

[Compare Source](https://renovatebot.com/gh/pytest-dev/pluggy/compare/0.7.1...0.8.0)

=========================

## Features

-   `#&#8203;177 <https://github.com/pytest-dev/pluggy/issues/177>`\_: Add `get_hookimpls()` method to hook callers.

## Trivial/Internal Changes

-   `#&#8203;165 <https://github.com/pytest-dev/pluggy/issues/165>`\_: Add changelog in long package description and documentation.

-   `#&#8203;172 <https://github.com/pytest-dev/pluggy/issues/172>`\_: Add a test exemplifying the opt-in nature of spec defined args.

-   `#&#8203;57 <https://github.com/pytest-dev/pluggy/issues/57>`\_: Encapsulate hook specifications in a type for easier introspection.

</details>

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frozen_requirements.txt

@@ -77,7 +77,7 @@ pager==3.3
 paramiko==2.4.2
 pefile==2018.8.8
 pkginfo==1.4.2
-pluggy==0.7.1
+pluggy==0.8.0
 pre-commit==1.11.2
 prettytable==0.7.2
 py==1.7.0