v1.3.3 and v1.3.4 is not compatible with py38/py39

Despite the fact that Cerberus==1.3.4 list compatibility with py39 we can easily get:

.tox/py39/lib/python3.9/site-packages/cerberus/validator.py:19: in <module>
    from cerberus import errors
E     File "/Users/ssbarnea/c/a/molecule/.tox/py39/lib/python3.9/site-packages/cerberus/errors.py", line 156
E       """
E       ^
E   SyntaxError: invalid escape sequence \*

While this bug was fixed in master it is not of any help for consumers of the library as there is no newer release. Even a pre-release like 2.0.0a0 could help as it would allow us to request it when running on py39 in setup.cfg.

Another approach would be backporting the fixed and making another hotfix release.

[tadeboro]

@funkyfuture Are you running flake8 from the repo? The invalid escape sequence will not be reported there because the tox.ini file instructs flake8 to ignore that error: https://github.com/pyeve/cerberus/blob/a70b404e5bf164bb6eaf1d7f264957db4ab7d55c/tox.ini#L28

That change was introduced in https://github.com/pyeve/cerberus/commit/e2c5620ad88e8e3f20edadb66d2c60793b44d338#diff-ef2cef9f88b4fe09ca3082140e67f5ad34fb65fb6e228f119d3812261ae51449, but I cannot find the reason behind the addition of that ignore entry.

[tadeboro]

The invalid escape sequence warning is silenced by default because it would annoy people maintaining ancient codebases written when those escape sequences were valid. Python interpreted them as literal backslash plus whatever comes after it. But “THe Right Way ™” that Python devs suggest here is to either use raw string literals or escape the backslash.