Fix ruff lint issues in Python code

[cclauss]

% ruff check --output-format=concise # https://docs.astral.sh/ruff/linter

src/cpulist.py:63:1: E401 [*] Multiple imports on one line
src/cpulist.py:69:5: F403 `from pyperf.perf_util import *` used; unable to detect undefined names
src/cpulist.py:371:39: F405 `list_cpusetstr` may be undefined, or defined from star imports
src/cpulist.py:372:39: F405 `list_cpusetstr` may be undefined, or defined from star imports
src/sysreport.py:36:1: E401 [*] Multiple imports on one line
src/sysreport.py:36:56: F401 [*] `json` imported but unused
src/sysreport.py:217:13: F841 Local variable `hdr` is assigned to but never used
src/sysreport.py:511:9: E401 [*] Multiple imports on one line
Found 8 errors.
[*] 4 fixable with the `--fix` option (1 hidden fix can be enabled with the `--unsafe-fixes` option).