pip-requirements.txt: Update to edk2-pytool-extensions 0.20.0

pip-requirements.txt: Update to edk2-pytool-extensions 0.20.0

Updates edk2-pytool-extensions to pick up a major version release:

0.19.1 to 0.20.0 changes:

- .vscode/settings.json: Enable flake8 linting
- Add Pydocstyle
- Move dependabot.yml location
- Fix typos in robot files
- Pydocstyle Updates
- Plugin Loader Updates
- edk2_stuart_pr_eval: Improve robustness of path comparisons
- edk2_pr_eval.py: Build all packages on file change outside package
- Allow build wrappers

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Sean Brogan <sean.brogan@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>