Replace obsolete `popen2`

[DimitriPapadopoulos]

https://github.com/Blosc/python-blosc/blob/20f7821267c053a8c3a2a7569bfd3c74e7daa64c/blosc/toplevel.py#L43-L44

See:

• popen2 — Subprocesses with accessible I/O streams
• Replacing Older Functions with the subprocess Module.

Additionally, avoid starting a process using a shell, as is probably the case here (not sure about macOS).