Add unit tests for internal retry decorator
Add support for socket timeout retries in scc

Fix #48
Add support for SSL error retries in scc
Refacotr TestInternalRetries to run series of tests on each class
Add test calss for GithubRepository __init__ method retries
Decorate GitHubRepository get_pull, get_issue and get_pulls methods

Implement corresponding unit tests
Add internal retries test for GHManager.get_organization()
Decorate GHManager.get_login()
Decorate additional PullRequest methods with retry_on_error
Use Github.get_repo() method to initialize GH repos and apply decorator

Add internal retry tests for GHManager.get_repo() and fix GithubRepository
mock calls for initialization
Fix flake8 warnings
Decorate open_pr() method with retry decorator
Add get_pulls_by_base() method and decorate it with retry_on_error
Allow to set the number of retries by environment variable
Fix flake8 warnings
Refactor Label command to use PullRequest lofic
Add 405 to the list of retry codes for Github exceptions
Use unique identifier for conflicting rebase integration test
Reuse PullRequest create_comment method in TestUnrebasedPRs
Rename create_comment() as create_issue_comment() for clarity

- Wrap scc rebase prs using PullRequest objects
- Fix PullRequest unit tests