Allow use of pathlib.Path objects for custom_schema option

[drdavella]

This fixes #653.

[stsci-bot]

Hi there @drdavella 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog. I help make sure that this pull request is milestoned and labeled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

Everything looks good from my point of view! 👍

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[codecov]

Codecov Report

Merging #663 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #663   +/-   ##
=======================================
  Coverage   93.24%   93.24%           
=======================================
  Files          39       39           
  Lines        4295     4295           
=======================================
  Hits         4005     4005           
  Misses        290      290

Impacted Files	Coverage Δ
asdf/schema.py	94.16% <100%> (ø)	⬆️

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