Change setup.py to use dependency markers.
Change setup.py to use dependency markers.

- Require enum34 only on Python versions earlier than 3.4.
- Use platform_system marker for OS-specific dependencies instead of
  modifying the install_requires list.

mbedmicro/pyOCD,mesheven/pyOCD,mbedmicro/pyOCD,pyocd/pyOCD,mesheven/pyOCD,mesheven/pyOCD,flit/pyOCD,pyocd/pyOCD,flit/pyOCD,mbedmicro/pyOCD