Remove syntax incompatible with python 2

[HoussemNasri]

Closes #1373

Result of python -m compileall . ran with python 2.7.13 inside the atlassian directory

Listing . ...
Compiling .\__init__.py ...
Compiling .\bamboo.py ...
Listing .\bitbucket ...
Compiling .\bitbucket\__init__.py ...
Compiling .\bitbucket\base.py ...
Listing .\bitbucket\cloud ...
Compiling .\bitbucket\cloud\__init__.py ...
Compiling .\bitbucket\cloud\base.py ...
Listing .\bitbucket\cloud\common ...
Compiling .\bitbucket\cloud\common\__init__.py ...
Compiling .\bitbucket\cloud\common\builds.py ...
Compiling .\bitbucket\cloud\common\comments.py ...
Compiling .\bitbucket\cloud\common\users.py ...
Listing .\bitbucket\cloud\repositories ...
Compiling .\bitbucket\cloud\repositories\__init__.py ...
Compiling .\bitbucket\cloud\repositories\branchRestrictions.py ...
Compiling .\bitbucket\cloud\repositories\commits.py ...
Compiling .\bitbucket\cloud\repositories\defaultReviewers.py ...
Compiling .\bitbucket\cloud\repositories\deploymentEnvironments.py ...
Compiling .\bitbucket\cloud\repositories\diffstat.py ...
Compiling .\bitbucket\cloud\repositories\groupPermissions.py ...
Compiling .\bitbucket\cloud\repositories\hooks.py ...
Compiling .\bitbucket\cloud\repositories\issues.py ...
Compiling .\bitbucket\cloud\repositories\pipelines.py ...
Compiling .\bitbucket\cloud\repositories\pullRequests.py ...
Compiling .\bitbucket\cloud\repositories\refs.py ...
Compiling .\bitbucket\cloud\repositories\repositoryVariables.py ...
Listing .\bitbucket\cloud\workspaces ...
Compiling .\bitbucket\cloud\workspaces\__init__.py ...
Compiling .\bitbucket\cloud\workspaces\members.py ...
Compiling .\bitbucket\cloud\workspaces\permissions.py ...
Compiling .\bitbucket\cloud\workspaces\projects.py ...
Listing .\bitbucket\server ...
Compiling .\bitbucket\server\__init__.py ...
Compiling .\bitbucket\server\base.py ...
Listing .\bitbucket\server\common ...
Compiling .\bitbucket\server\common\__init__.py ...
Compiling .\bitbucket\server\common\permissions.py ...
Compiling .\bitbucket\server\globalPermissions.py ...
Listing .\bitbucket\server\projects ...
Compiling .\bitbucket\server\projects\__init__.py ...
Listing .\bitbucket\server\projects\repos ...
Compiling .\bitbucket\server\projects\repos\__init__.py ...
Compiling .\cloud_admin.py ...
Compiling .\confluence.py ...
Compiling .\crowd.py ...
Compiling .\errors.py ...
Compiling .\insight.py ...
Compiling .\jira.py ...
Compiling .\marketplace.py ...
Compiling .\portfolio.py ...
Compiling .\request_utils.py ...
Compiling .\rest_client.py ...
Compiling .\service_desk.py ...
Compiling .\statuspage.py ...
Compiling .\utils.py ...
Compiling .\xray.py ...