Remove fsdecode() calls for subprocess commands

[rmartin16]

Changes

• Removes calls to os.fsdecode() for command arguments for subprocess calls
• subprocess will call fsdecode() for subprocess command arguments for us
  • Windows: subprocess always calls list2cmdlist for Windows processes
  • POSIX: for posix_spawn path or the fork_exec path, everything has fsdecode called

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

This all makes sense; the RTD build failure looks to be unrelated. I've pushed #1695 to correct this; once that lands we can rebase and get a clean board here. Approving on that basis; as soon as there's a clean CI run, I'm OK to merge.