[BUG]: Fix errors from `-Wsign-compare` in Python bindings

[jhale]

Summarize the issue

Further use of strict compiler flags across the CI led to a number of -Wsign-compare errors being triggered when building the Python bindings.

These are currently being ignored:

https://github.com/FEniCS/dolfinx/pull/4051/changes#diff-005a07ca82fb46b215f58a3a8f4d6d6cc6ca088ade44e8f15cc2e6a51d1da6f4R87

and should be fixed.

How to reproduce the bug

Remove -Wno-sign-compare from python/CMakeLists.txt and build DOLFINx Python bindings in Developer mode

Minimal Example (Python)

Output (Python)

Version

main branch

DOLFINx git commit

No response

Installation

No response

Additional information

No response