Add support for pathlib.Path file paths

[zoquda]

Description

Currently the xlwings API does not seem to support file paths that are formatted as pathlib.Path object. The following code example demonstrates this:

# OS: Windows 10
# xlwings: 0.15.8
# excel: 2013
# python: 3.7.3

from pathlib import Path
import xlwings as xw

# Create dummy excel file
book = xw.Book()
book.save('example_file.xlsx')
book.close()

# Attempt to connect to excel file using Pathlib path
file_path = Path('example_file.xlsx').resolve()
xw.Book(file_path)

The subsequent error traceback is:

AttributeError                            Traceback (most recent call last)
~\Desktop\example.py in <module>
      9 # Attempt to connect to excel file using Pathlib path
     10 file_path = Path('example_file.xlsx').resolve()
---> 11 xw.Book(file_path)

C:\Software\Anaconda3\lib\site-packages\xlwings\main.py in __init__(self, fullname, impl)
    474         if not impl:
    475             if fullname:
--> 476                 fullname = fullname.lower()
    477
    478                 candidates = []

AttributeError: 'WindowsPath' object has no attribute 'lower'

The addition of support for pathlib.Path file paths is hereby proposed as enhancement of xlwings.

Rationale

Use of pathlib.Path objects for file paths is attractive to users because (compared to string object file paths) they are usually easier to work with and more consistent across operating systems (see Python 3's pathlib Module: Taming the File System for more details). The pathlib module is readily available, since it is part of python's standard library since version 3.4.