Make _DictManager private.

[bdice]

Description

This PR makes _DictManager private. Addresses a TODO in the code.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• The changes introduced by this pull request are covered by existing or newly introduced tests.
• The package documentation and framework documentation in signac-docs are up to date with these changes.
• I have updated the changelog and added any related issue and pull request numbers for future reference.

[codecov]

Codecov Report

Merging #858 (6f9ba48) into master (bd65753) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #858   +/-   ##
=======================================
  Coverage   85.07%   85.07%           
=======================================
  Files          50       50           
  Lines        4669     4669           
  Branches      924      924           
=======================================
  Hits         3972     3972           
  Misses        518      518           
  Partials      179      179           

Impacted Files	Coverage Δ
signac/core/dict_manager.py	88.37% <100.00%> (ø)
signac/core/h5store.py	91.03% <100.00%> (ø)

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