package/python-fire.mk: drop test_components_py3.py file for python 2.x to fix pyfile...

package/python-fire.mk: drop test_components_py3.py file for python 2.x to fix pyfile issue

Fixes:
http://autobuild.buildroot.net/results/72e0cc78194a1b93bf26a50742e59a1e93bde1d1/

fire has conditional logic to load test_components_py3.py when running under
Python 3.x:

if six.PY3:
  from fire import test_components_py3 as py3

pycompile unfortunately errors out on it:

../scripts/pycompile.py ..

error:   File "/usr/lib/python2.7/site-packages/fire/test_components_py3.py", line 18
    def identity(arg1, arg2: int, arg3=10, arg4: int = 20, *arg5,
                           ^
SyntaxError: invalid syntax

As a workaround, simply drop the unusable _py3 file from TARGET_DIR if
building for python 2.x.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>