* BUG: Avoid BrokenProcessPool error on Windos
* REF: Avoid any multiprocessing exceptions in `Backtest.optimize()`
... and instead switch to non-parallel computation.
* REF: Support parallelism only if start method is 'fork'
* REF: In case of non-'fork' multiprocessing, use a simple loop
* MNT: quiet codecov
* TST: Remove redundant test
The code path is covered by any other Backtest.optimize() test.
* fix flake8 warning