Read Python3 Source File With Correct Encoding

according to https://www.python.org/dev/peps/pep-0263/, python3 source file encoding are default to be utf-8.

But open()'s encoding will be gbk on windows, So don't use open without encoding to read a python3 source file.

example:

from read_source import get_encoding, read
print(get_encoding('tests/source/gb18030/dash-star-dash.py')) # gb18030
with read('tests/source/gb18030/dash-star-dash.py') as f:
    print(f.read()) 
    # -*- coding: gb18030 -*-