drop Python 2.7 support

Python 2.7 EOL is arriving on 2020-01-01: https://pythonclock.org/

This will allow us to use Python 3 only libraries, like python-barcode,
which can maintain a reduced, simpler codebase, due to only one version
to support.

Closes #371.

Signed-off-by: Romain Porte <microjoe@microjoe.org>

test/test_function_barcode.py

@@ -1,8 +1,4 @@
 #!/usr/bin/python
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-from __future__ import unicode_literals
 
 import escpos.printer as printer
 from escpos.constants import BARCODE_TYPE_A, BARCODE_TYPE_B