Remove docs build dependencies from packaged Kart

[craigds]

Describe the bug
Packaged kart contains dependencies that should only be installed during CI when we build the docs, and shouldn't be in the packages.

To Reproduce
Steps to reproduce the behaviour:

1. run kart --post-mortem x with a packaged Kart
2. type import requests into the pdb prompt
3. it works 👎

Expected behaviour
requests (and other docs build dependencies in docs.txt) shouldn't be included in kart packages

Output

$ kart --post-mortem x
> /app/click/core.py(679)fail()
(Pdb) import requests
(Pdb)

**Version Info **

• OS: ubuntu 18.04
• Version: v0.13.1.dev0+ci.5414.git889a47e0