Upgrade dependencies in requirements.txt

Description

The type check fails when using the fully pinned dependencies (requirements/requirements.txt) which can be reproduced by

# a new python venv
pip install -r requirements/requirements.txt
mypy --install-types --non-interactive --namespace-packages buildgrid/

To fix this, we need to upgrade the full dependency graph.

Changes proposed in this merge request:

• Remove unused requirements.*.txt files
• Update the manage-deps script so it can upgrade requirements.txt properly
• Upgrade requirements.txt
• Use backtracking dependency resolver which should be more robust than the legacy resolver
  • docs from pip-tools
  • docs from pip