Chore: Fix `ResourceWarning`s from tests

[mmore500]

• [ ] .

  home/runner/work/DendroPy/DendroPy/tests/test_phylogenetic_distance_matrix.py:829: ResourceWarning: unclosed file <_io.TextIOWrapper name='/home/runner/work/DendroPy/DendroPy/tests/support/../data/other/hiv1.node-to-node-dists.csv' mode='r' encoding='UTF-8'>
  delimiter=",")

• [ ] .

  /home/runner/work/DendroPy/DendroPy/tests/test_phylogenetic_distance_matrix.py:829: ResourceWarning: unclosed file <_io.TextIOWrapper name='/home/runner/work/DendroPy/DendroPy/tests/support/../data/other/pythonidae.mle.node-to-node-dists.csv' mode='r' encoding='UTF-8'>
  delimiter=",")

• [ ] .

  /home/runner/work/DendroPy/DendroPy/tests/test_phylogenetic_distance_matrix.py:829: ResourceWarning: unclosed file <_io.TextIOWrapper name='/home/runner/work/DendroPy/DendroPy/tests/support/../data/other/hiv1.unweighted.node-to-node-dists.csv' mode='r' encoding='UTF-8'>
  delimiter=",")

• [ ] .

  /home/runner/work/DendroPy/DendroPy/tests/test_phylogenetic_distance_matrix.py:829: ResourceWarning: unclosed file <_io.TextIOWrapper name='/home/runner/work/DendroPy/DendroPy/tests/support/../data/other/pythonidae.mle.unweighted.node-to-node-dists.csv' mode='r' encoding='UTF-8'>
  delimiter=",")

• [ ] .

  /opt/hostedtoolcache/Python/3.5.10/x64/lib/python3.5/unittest/case.py:605: ResourceWarning: unclosed file <_io.TextIOWrapper name='/home/runner/work/DendroPy/DendroPy/tests/support/../data/other/birth-death-test-data1.json' mode='r' encoding='UTF-8'>
  testMethod()