Set upper bound on jinja2 dependency

[drewbanin]

Describe the feature

At the time of this writing, jinja2 v3.0.0a1 is now available on PyPi. This version is not compatible with dbt.

We should either:

• set the upper bound on the jinja2 dependency to be <3.0.0, or
• make dbt compatible with version 3.0.0 when it is released (likely as a part of a future issue)

The proximate error here looks like:

No module named 'jinja2._compat'

as far as I can tell.

[beckjake]

I am pretty sure that since we dropped python 2.x support, we don't need that call anymore.