Drop manifest and restore some of conftest.py to ccdproc

[mwcraig]

Please have a look at the following list and replace the "[ ]" with a "[x]" if
the answer to this question is yes.

• For new contributors: Did you add yourself to the "Authors.rst" file?

For documentation changes:

• For documentation changes: Does your commit message include a "[skip ci]"?
  Note that it should not if you changed any examples!

For bugfixes:

• Did you add an entry to the "Changes.rst" file?
• Did you add a regression test?
• Does the commit message include a "Fixes #issue_number" (replace "issue_number").
• Does this PR add, rename, move or remove any existing functions or parameters?

For new functionality:

• Did you add an entry to the "Changes.rst" file?
• Did you include a meaningful docstring with Parameters, Returns and Examples?
• Does the commit message include a "Fixes #issue_number" (replace "issue_number").
• Did you include tests for the new functionality?
• Does this PR add, rename, move or remove any existing functions or parameters?

Please note that the last point is not a requirement. It is meant as a check if
the pull request potentially breaks backwards-compatibility.

---

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.87%. Comparing base (f2d93de) to head (211d9ef).
Report is 25 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #884   +/-   ##
=======================================
  Coverage   97.87%   97.87%           
=======================================
  Files           8        8           
  Lines        1362     1362           
=======================================
  Hits         1333     1333           
  Misses         29       29           

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