Add issue dependency endpoints (#3477)

This PR adds four new methods to the Issue class to interact with the
REST API endpoints for interacting with issue dependencies. This would
close #3397.

## Changes

`get_blocked_by` returns paginated list of issues that the calling issue
is blocked by.

`add_blocked_by` accepts an Issue object and adds it as blocking to the
calling issue.

`remove_blocked_by` accepts an Issue object and removes it as blocking
to the calling issue.

`get_blocking` returns paginated list of issues that the calling issue
is blocking.

## Discussion

Because issues from other repositories can be blocking, both the add and
remove endpoints require the unique issue ID (versus an issue number
within the calling repository).

## Example Usage

```python
auth = Auth.Token("")

g = Github(auth=auth)

repo = g.get_repo('PyGithub/PyGithub')

i = repo.get_issue(96)
j = repo.get_issue(28)

i.add_blocked_by(j)
# returns None

j.get_blocking()
# returns paginated list containing i
```

---------

Co-authored-by: Enrico Minack <github@enrico.minack.dev>