Support utf8 mode

[psf]

Support utf8 mode

[vstinner]

pyperf removes most env vars to make benchmarks more reproducible between two machines. If you want to opt-in for the Python UTF-8 Mode on purpose, IMO it's good that pyperf forces you to declare it. Otherwise, you may get a different performance on two machines without knowning it.

But @corona10, @pablogsal or @sir-sigurd may have a different opinion ;-)

Maybe pyperf could get a more generic command line option to blindly copy all env vars, like "--copy-env-vars".

[pablogsal]

But @corona10, @pablogsal or @sir-sigurd may have a different opinion ;-)

I am of the same opinion :)

[corona10]

I am of the same opinion too ;)

[vstinner]

If someone wants a new "--copy-env-vars" option, feel free to write a PR for that, I will review it.