Create inactivity reminder bot for assigned issues with no pull requests

[exploreriii]

PROBLEM
Sometimes we have issues that are open and have someone assigned, but there is no pull request linked - probably meaning the developer has not started working on the issue at all.

SOLUTION
Create an automated reminder on issues with someone assigned that have no linked pull request.
It could trigger after 7 days.

it should be eg
"Hi, this is ReminderBot. This issue has been assigned but has had no pull request created Are you still planning on working on the issue?
From the Python SDK Team"

[exploreriii]

@prajeeta15 would this issue interest you? Ideally we find a way of testing it 👍

[prajeeta15]

cool will look into it too.

[prajeeta15]

@exploreriii sorry it took me time to get to it, was sorta busy with tests.