Fix decode error of README.rst during installation

Fix decode error in environment that default encoding is not utf-8.

Error log:
```
Traceback (most recent call last):
  File "<string>", line 17, in <module>
  File "C:\Users\Asvel\AppData\Local\Temp\pip_build_Asvel\tinydb\setup.py", line 36, in <module>
	long_description=open('README.rst', 'r').read(),
UnicodeDecodeError: 'gbk' codec can't decode byte 0x93 in position 1037: illegal multibyte sequence
```