Support Python 2.7

One extra dependency is pulled in for Python versions older than 3.6,
but other than that it's pretty easy to support them.  It definitely
works on 2.7 now, and should work on older 3.x versions (but that's
untested).

README.rst

 Requirements
 ------------
 
--  Python >= 3.6
+-  Python 2.7, 3.6
 -  pySerial >= 3.0
+-  aenum >= 2.0 (if using Python < 3.6)

pdbuddy/__init__.py

 """Python bindings for PD Buddy Sink configuration"""
 
-from enum import *
+try:
+    # Try importing enum from the standard library
+    import enum
+    # Make sure Flag is available
+    enum.Flag
+except (ImportError, NameError):
+    # If something above failed, try aenum instead
+    import aenum as enum
 
 import serial
 import serial.tools.list_ports
             printed as a response to the command.
         """
         # Send the command
-        self._port.write(bytes(cmd, "utf-8") + b"\r\n")
+        self._port.write(cmd.encode("utf-8") + b"\r\n")
         self._port.flush()
 
         # Read the result
         self.i = i
 
     def __repr__(self):
-        s = type(self).__name__ + "("
+        s = self.__class__.__name__ + "("
 
         if self.status is not None:
             s += "status={}".format(self.status)
         return cls(status=status, flags=flags, v=v, i=i)
 
 
-class SinkStatus(Enum):
+class SinkStatus(enum.Enum):
     """Status field of a PD Buddy Sink configuration object"""
     EMPTY = 1
     VALID = 2
     INVALID = 3
 
 
-class SinkFlags(Flag):
+class SinkFlags(enum.Flag):
     """Flags field of a PD Buddy Sink configuration object"""
     NONE = 0
-    GIVEBACK = auto()
+    GIVEBACK = enum.auto()