How to get the host name of the current machine as defined in the Ansible hosts file?

The necessary variable is inventory_hostname.

- name: Install this only for local dev machine
  pip: name=pyramid
  when: inventory_hostname == "local"

It is somewhat hidden in the documentation at the bottom of this section.

You can limit the scope of a playbook by changing the hosts header in its plays without relying on your special host label ‘local’ in your inventory. Localhost does not need a special line in inventories.

- name: run on all except local
  hosts: all:!local