Pin pip to <26 due to incompatibility with pip-tools

[bmtcril]

Several repos currently have failing requirement upgrades due to the new pip-tools not being compatible with the newly released pip 26: jazzband/pip-tools#2319

It doesn't seem like they are moving fast on that issue so we may need to pin pip temporarily to work around it.

[bmtcril]

This is now fixed in 7.5.3: https://github.com/jazzband/pip-tools/releases/tag/v7.5.3 and we should be able to unpin things.