add Stickler CI config

[wholmgren]

I'd like to try using Stickler CI service with pvlib. https://stickler-ci.com

Stickler CI will automatically review pull requests and comment on style issues. I configured it on my fork and made this example pull request: wholmgren#5

Pros:

• Maintainers don't have to spend as much time on style issues
• It won't accidentally overlook issues
• People can get frustrated at a bot that nags them instead of at me

Cons:

• It's an extra notification clogging your inbox
• Maybe it makes pull requests a little harder to read