Use MacOS 10.15 instead of MacOS 10.13

[rly]

Similar to hdmf-dev/hdmf#310

This fixes the issue that will break Azure Pipelines CI on March 23.

This pipeline uses a Microsoft-hosted agent image that will be removed on March 23, 2020 (MacOS-10.13). You must make changes to your pipeline before that date, or else your pipeline will fail. Learn more (https://aka.ms/removing-older-images-hosted-pools).

[codecov]

Codecov Report

Merging #1211 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1211   +/-   ##
=======================================
  Coverage   68.01%   68.01%           
=======================================
  Files          37       37           
  Lines        2401     2401           
  Branches      423      423           
=======================================
  Hits         1633     1633           
  Misses        697      697           
  Partials       71       71

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