include prs with stalebnot

[GregTheGreek]

Description

Stalebot was previously only configured for issues, figured its worth adding to PR's so we can keep track of contributions that might have gotten stale/out-dated

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run dtslint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success.
• I ran npm run test:cov and my test cases cover all the lines and branches of the added code.
• I ran npm run build and tested dist/web3.min.js in a browser.
• I have tested my code on the live network.
• I have checked the Deploy Preview and it looks correct.
• I have updated the CHANGELOG.md file in the root folder.

[coveralls]

Coverage remained the same at 84.321% when pulling a92441f on greg/ci/stale-bot-pr into 1fc6217 on 1.x.

[ryanio]

Lgtm!