update documentation string in pexpect/__init__.py to be compatible with...

Update documentation string in pexpect/__init__.py to be compatible with python 2 and 3.
With the examples in the previous doc string running in Python 3, without changing file open mode from 'w' to 'wb' or using spawnu instead of spawn for logging to stdout, the following exception will be raised:
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.4/site-packages/pexpect-3.2-py3.4.egg/pexpect/__init__.py", line 1418, in expect
  File "/usr/lib/python3.4/site-packages/pexpect-3.2-py3.4.egg/pexpect/__init__.py", line 1433, in expect_list
  File "/usr/lib/python3.4/site-packages/pexpect-3.2-py3.4.egg/pexpect/__init__.py", line 1502, in expect_loop
  File "/usr/lib/python3.4/site-packages/pexpect-3.2-py3.4.egg/pexpect/__init__.py", line 926, in read_nonblocking
  File "/usr/lib/python3.4/site-packages/pexpect-3.2-py3.4.egg/pexpect/__init__.py", line 847, in _log
TypeError: must be str, not bytes
```
The updated doc string will be more user-friendly to Python 3 users.