Use requirements.txt as the definitive source of packages to install.

[dumol]

Through #58, we started using requirements.txt instead of safety for alerting us of security issues, leveraging GitHub's own security checks.

As suggested at #58 (comment), we could use it as the definitive source of packages to install, e.g.:

pip==24.2
psutil==6.0.0
pycparser==2.22
pywin32==306; sys_platform == 'windows'

This should work better than defining the packages to install through build.conf, then auto-updating requirements.txt with pip freeze, and then automatically commiting the update at the end of a successful Windows build.