style: fix missing whitespace between operators

[echoix]

In between the last time I applied the missing pre-commit violations and the time it was merged, some new violations were introduced in here:
a24714e#diff-fdcb5f33abc00bc15a934808d3d868490e61db92909e6747a2a635faf2c4df02R20
as the PR changed python files, but the CI wasn't run containing the changes on main that updated the flake8 version or exclusions.

Anyways, here's a missing fix for this:

gitpod /workspace/grass $ flake8 --count --statistics --show-source --jobs=$(nproc) .
./raster/r.texture/benchmark/benchmark_rtexture.py:20:62: E226 missing whitespace around arithmetic operator
        benchmark(int(mapsize**0.5), f"r.texture_{int(mapsize/1e6)}M", results)
                                                             ^
1     E226 missing whitespace around arithmetic operator
1