Add pathlib support to Database reading

[bocklund]

Enables the following behavior:

from pathlib import Path
from pycalphad import Workspace
wks = Workspace()
wks.database = Path("dat/Fe-C.dat")

Closes #544

We test this by changing the tests that usually wrap files() output (a PosixPath) in a str() to make a string-like path. We remove this behavior in several tests.

This also changes some tests (test_unspecified_format_from_string and test_unspecified_format_from_file) to be more in line with what I interpreted to be the spirit of the tests: passing database strings (not strings for the paths to the databases) to Database.from_string or Database.from_file should fail if a format is not specified.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.62%. Comparing base (8595b84) to head (921d6ca).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #545   +/-   ##
========================================
  Coverage    91.62%   91.62%           
========================================
  Files           60       60           
  Lines         9918     9924    +6     
========================================
+ Hits          9087     9093    +6     
  Misses         831      831           

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