Doesn't work with Python 3.4

[rgerganov]

• YubiKey Manager (ykman) version:
  top-of-tree
• How was it installed?:
  pip install -e .
• Operating system and version:
  Ubuntu 14.04
• YubiKey model and version:
  Yubikey 5 NFC
• Bug description summary:

Steps to reproduce

ykman info

Expected result

To get info for the key that is plugged

Actual results and logs

Device type: YubiKey 5 NFC
Serial number: 9261996
Firmware version: 5.1.2
Traceback (most recent call last):
  File "venv/bin/ykman", line 11, in <module>
    load_entry_point('yubikey-manager', 'console_scripts', 'ykman')()
  File "/opt/src/yubikey-manager/ykman/cli/__main__.py", line 246, in main
    cli(obj={})
  File "/opt/src/yubikey-manager/venv/lib/python3.4/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/opt/src/yubikey-manager/venv/lib/python3.4/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/opt/src/yubikey-manager/venv/lib/python3.4/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/opt/src/yubikey-manager/venv/lib/python3.4/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/opt/src/yubikey-manager/venv/lib/python3.4/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/opt/src/yubikey-manager/venv/lib/python3.4/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/opt/src/yubikey-manager/ykman/cli/info.py", line 154, in info
    click.echo('Form factor: {!s}'.format(config.form_factor))
  File "/opt/src/yubikey-manager/ykman/util.py", line 116, in __str__
    if self == self.USB_A_KEYCHAIN:
AttributeError: 'FORM_FACTOR' object has no attribute 'USB_A_KEYCHAIN'

Other info

I've fixed the issue and will submit a PR