Fix python3 compatibility issue.

[Mason-Lin]

Hi tjquk, First of all, thank you for taking the time to review and maintain this project. I found some issue while I trying to use _wmi_class's to_csv() function in python3.

1. TypeError: a bytes-like object is required, not 'str'

• I modify "wb" to "w".
• reference

  >>> system.Win32_ComputerSystem.to_csv()
  Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\mason\fix-wmi\wmi\wmi.py", line 860, in to_csv
  writer.writerow(fields)
  TypeError: a bytes-like object is required, not 'str'
  >>>

2. NameError: name 'unicode' is not defined

• I delete _to_utf8() and open with encoding="utf-8"

• reference

  >>> system.Win32_ComputerSystem.to_csv()
  Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\mason\fix-wmi\wmi\wmi.py", line 862, in to_csv
  writer.writerow([_to_utf8(getattr(instance, field)) for field in fields])
  File "C:\Users\mason\fix-wmi\wmi\wmi.py", line 862, in <listcomp>
  writer.writerow([_to_utf8(getattr(instance, field)) for field in fields])
  File "C:\Users\mason\fix-wmi\wmi\wmi.py", line 851, in _to_utf8
  if isinstance(item, unicode):
  NameError: name 'unicode' is not defined
  >>>

  Hope you can take a review when you have time. Thank you.

Warm regards, Mason