Conform to Puppet style guide

Fixes: puppet-lint "ERROR: two-space soft tabs not used" error
Fix regression with $cwd in python::requirements

Commit 8b22e3ecd90dd3ac741ff107d6f3fc8c511443ba introduces a regression
which in some cases specifies the full path to pip as it's current
working directory, rather than the actual directory pip is in. Obviously
this will fail.

This commit fixes this.