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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_travis_branch():
 """Get current branch per Travis environment variables If travis is building a PR, then TRAVIS_PULL_REQUEST is truthy and the name of the branch corresponding to the PR is stored in the TRAVIS_PULL_REQUEST_BRANCH environment variable. Else, the name of the branch is stored in the TRAVIS_BRANCH environment variable. See also: <https://docs.travis-ci.com/user/environment-variables/#default-environment-variables> """
  # noqa E501
    try:
        travis_pull_request = get_travis_env_or_fail('TRAVIS_PULL_REQUEST')
        if truthy(travis_pull_request):
            travis_pull_request_branch = get_travis_env_or_fail(
                'TRAVIS_PULL_REQUEST_BRANCH')
            return travis_pull_request_branch
        else:
            travis_branch = get_travis_env_or_fail('TRAVIS_BRANCH')
            return travis_branch
    except UnexpectedTravisEnvironmentError:
        return None