Fix some tshark_path problems.

[miaotony]

Get config.ini from the current directory first, then the pyshark module directory.
Check if the file exists before reading it.

In the src/pyshark/tshark/tshark.py, also check if config.ini exists before adding it to possible_paths.

The priority that tshark_path loads will be:
user provided tshark_path > config.ini in the current directory > config.ini in the pyshark module directory > system env

This PR will fix the issue #564 .

[miaotony]

I have used pathlib instead of os.path in config.py.

But you have to pay attention to the different behaviors between PurePath.parent and os.path.dirname().

Just like below.

>>> os.path.dirname(r'C:\adsf\\asdfd\\')
'C:\\adsf\\\\asdfd'
>>> os.path.dirname(r'C:\adsf\\asdfd\\adsf.exe')
'C:\\adsf\\\\asdfd'
>>> Path(r'C:\adsf\\asdfd\\').parent
WindowsPath('C:/adsf')
>>> Path(r'C:\adsf\\asdfd\\adsf.exe').parent
WindowsPath('C:/adsf/asdfd')