There's an assert in the math.isqrt equivalent Python code (which is in a comment in Modules/mathmodule.c). This trivial PR changes that assert to a comment in order to:

• better clarify the intent of the original assert (it's a loop invariant)
• better match the C code (which has no such assertion in it)
• avoid copy-and-pasted Python code being unnecessarily slow - the checks in the assert could easily dominate the running time

Also adds redundant parentheses for readability.