Do perturb shift after first conflict.

Issue #23749: With this change it's possible to implement starttls
as a separate package on PyPI, or even by copying/pasting a small
snipped of code in your project.

It's expected that we'll figure out the API design for starttls
during 3.6, so that we can add it in 3.7.

Patch by Коренберг Марк.

Patch by Vincent Michel.

os.walk() function and glob module since os.scandir() now directly supports
them.

We can't just skip the test if docutils is available,
but pygments is not because the purpose of the test
was testing a bug in _check_rst_data().

Patch by Mariatta Wijaya.