Use relative imports

[CendioOssman]

This makes it possible to embed Cheetah in other packages without risking conflict with other versions.

[phdru]

Thank you! There is a minor failing flake8 test under Python 2.7. See https://app.travis-ci.com/github/CheetahTemplate3/cheetah3/jobs/525956382. Can you please fix it?

My advice is to make the code like this:

package = __spec__.parent  # noqa: F821 undefined name '__spec__'

Please run all tests locally. Running tox is the simplest way.

[CendioOssman]

Sorry for the delay. I just got back from vacation.

Pushed a new version with a noqa for that line. Should hopefully make flake8 happier. It passes here locally now at least.

[phdru]

Merged. Thank you very much!