remove Pipenv in favor of pip-tools, security updates

[redshiftzero]

Closes #136
Closes #138

Testing

• Verify hashes are the same from the removed Pipfile.lock compared to requirements.txt except the one dependency updated here
• Verify dev setup works for you following README updates
• Verify static analysis, CVE safety check, test jobs pass in CI

Required diff review

• urllib3 1.24 -> 1.24.3 (see if this was done in remove Pipenv in favor of pip-tools, security updates  securedrop-client#372, same diff update)

Check prior to merge

• PR is made and approved over in securedrop-client: remove Pipenv in favor of pip-tools, security updates  securedrop-client#372
• PR is made and approved over in securedrop-debian-packaging

^ once all these PRs are approved, I'll take this off of draft mode

[kushaldas]

Reviewed urllib3 1.24.3 https://github.com/freedomofpress/securedrop-debian-packaging/wiki/urllib3-1.24.3

[kushaldas]

This looks good. Approved. 🦄 🦄 🦄 🦄