fix: stub heavy imports during shell completion

[seroperson]

Closes #703

Summary

Completions were introduced in #431 and seemd work quite fast, but currently they're noticeable slow. You can check it either right during completion attempt or by running:

$ time _GITT_COMPLETE=zsh_complete COMP_WORDS="gitt " COMP_CWORD=1 gitt >/dev/null
0.711 total

The reason is heavy bittensor import which was introduced to the main.py by #637:

# added in #637:
from gittensor.utils.github_api_tools import make_graphql_headers, make_headers

# gittensor/utils/github_api_tools.py:
import bittensor as bt

Such heavy-imports issue is already solved in allways (entrius/allways#45), so looks like it's time to replicate the same fix here.

After

$ time _GITT_COMPLETE=zsh_complete COMP_WORDS="gitt " COMP_CWORD=1 gitt >/dev/null
0.081 total