Use select() syscall to avoid busy waiting

[jrfnl]

Note: this is a recreation of PR #110, which was closed due to the branch rename and the remote no longer existing. Please see PR #110 for the original discussion on the PR.

Currently it consumes a lot of CPU, it's a no-brainer i think.

/cc @diegode

[jrfnl]

@diegode Hiya!

We're currently planning two triage sessions for Requests 2.0 and would like to invite you to join us in one or both of these sessions to talk us through the PR and discuss it.

The triage sessions are currently planned for:

• Friday August 20, 07:00 - 11:00 UTC
• Friday September 3, 07:00 - 11:00 UTC

Would you be available to join us during those times on one of those days ?

[jrfnl]

Possible duplicate of #284 to fix #283