Exception occurs under Python 3.8

yapf seems not support python3.8

versions:

python -- 3.8.0
yapf -- 0.29.0

When I reformat the source tree which definitely no error, the following error message occurs:

```bash
$ yapf -pri .
concurrent.futures.process._RemoteTraceback:
"""
Traceback (most recent call last):
File "/usr/lib/python3.8/site-packages/yapf/yapflib/pytree_utils.py", line 115, in ParseCodeToTree
tree = parser_driver.parse_string(code, debug=False)
File "/usr/lib/python3.8/lib2to3/pgen2/driver.py", line 103, in parse_string
return self.parse_tokens(tokens, debug)
File "/usr/lib/python3.8/lib2to3/pgen2/driver.py", line 71, in parse_tokens
if p.addtoken(type, value, (prefix, start)):
File "/usr/lib/python3.8/lib2to3/pgen2/parse.py", line 162, in addtoken
raise ParseError("bad input", type, value, context)
lib2to3.pgen2.parse.ParseError: bad input: type=22, value='=', context=('', (25, 16))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/lib/python3.8/concurrent/futures/process.py", line 239, in _process_worker
r = call_item.fn(*call_item.args, **call_item.kwargs)
File "/usr/lib/python3.8/site-packages/yapf/__init__.py", line 298, in _FormatFile
reformatted_code, encoding, has_change = yapf_api.FormatFile(
File "/usr/lib/python3.8/site-packages/yapf/yapflib/yapf_api.py", line 85, in FormatFile
reformatted_source, changed = FormatCode(
File "/usr/lib/python3.8/site-packages/yapf/yapflib/yapf_api.py", line 129, in FormatCode
tree = pytree_utils.ParseCodeToTree(unformatted_source)
File "/usr/lib/python3.8/site-packages/yapf/yapflib/pytree_utils.py", line 121, in ParseCodeToTree
tree = parser_driver.parse_string(code, debug=False)
File "/usr/lib/python3.8/lib2to3/pgen2/driver.py", line 103, in parse_string
return self.parse_tokens(tokens, debug)
File "/usr/lib/python3.8/lib2to3/pgen2/driver.py", line 71, in parse_tokens
if p.addtoken(type, value, (prefix, start)):
File "/usr/lib/python3.8/lib2to3/pgen2/parse.py", line 162, in addtoken
raise ParseError("bad input", type, value, context)
lib2to3.pgen2.parse.ParseError: bad input: type=22, value='=', context=('', (25, 16))
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/usr/bin/yapf", line 11, in
load_entry_point('yapf==0.29.0', 'console_scripts', 'yapf')()
File "/usr/lib/python3.8/site-packages/yapf/__init__.py", line 344, in run_main
sys.exit(main(sys.argv))
File "/usr/lib/python3.8/site-packages/yapf/__init__.py", line 216, in main
changed = FormatFiles(
File "/usr/lib/python3.8/site-packages/yapf/__init__.py", line 274, in FormatFiles
changed |= future.result()
File "/usr/lib/python3.8/concurrent/futures/_base.py", line 432, in result
return self.__get_result()
File "/usr/lib/python3.8/concurrent/futures/_base.py", line 388, in __get_result
raise self._exception
lib2to3.pgen2.parse.ParseError: xxx/xxx.py
```