fix: validate urls for external accounts

[BigTailWolf]

[b-193694420] adding url validation for token url and service account impersonation url

• adding coverage of empty hostname to invalid token url
• Add more tests to enlarge the coverage
  Co-authored-by: Jin Qin qinjin@google.com

Blacken successful

❯ python -m nox -s blacken
nox > Running session blacken
nox > Creating virtual environment (virtualenv) using python3.8 in .nox/blacken
nox > python -m pip install click==8.0.4 black==19.3b0
nox > black google tests tests_async noxfile.py setup.py docs/conf.py
All done! ✨ 🍰 ✨
111 files left unchanged.
nox > Session blacken was successful.

Lint successful

❯ python -m nox -s lint
nox > Running session lint
nox > Creating virtual environment (virtualenv) using python3.8 in .nox/lint
nox > python -m pip install flake8 flake8-import-order docutils click==8.0.4 black==19.3b0
nox > python -m pip install -e .
nox > black --check google tests tests_async noxfile.py setup.py docs/conf.py
All done! ✨ 🍰 ✨
111 files would be left unchanged.
nox > flake8 --import-order-style=google --application-import-names=google,tests,system_tests google tests tests_async
nox > python setup.py check --metadata --restructuredtext --strict
running check
nox > Session lint was successful.

nox test added and 100% covered

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TOTAL                                              11158      0   1204      0   100%
nox > Session cover was successful.

[arithmetic1728]

Please change the commit message to "fix: ". The commit has to start with fix: prefix, otherwise the presubmit check will fail.

[arithmetic1728]

Unit test failed:

ImportError while importing test module '/tmpfs/src/github/google-auth-library-python/tests/test_identity_pool.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
tests/test_identity_pool.py:26: in <module>
    from google.auth import identity_pool
google/auth/identity_pool.py:47: in <module>
    from google.auth import external_account
google/auth/external_account.py:37: in <module>
    from urllib.parse import urlparse
E   ImportError: No module named parse

Lint nox session also failed. Please run python -m nox -s blacken to format the files, then python -m nox -s lint to make sure it can pass.

[lsirac]

LGTM, thanks Jin!

[arithmetic1728]

Session unit-3.6 failed
FAILED tests/test_external_account.py::TestCredentials::test_invalid_token_url_shall_throw_exceptions
FAILED tests/test_external_account.py::TestCredentials::test_invalid_service_account_impersonate_url_shall_throw_exceptions

for url in invalid_urls:
            # An invalid url should throw a ValueError exception
            with pytest.raises(ValueError) as excinfo:
                external_account.Credentials.validate_service_account_impersonation_url(
>                   url
                )
E               Failed: DID NOT RAISE <class 'ValueError'>