build: add diffpy.structure back to requirement

[stevenhua0320]

@sbillinge ready to review. As now we have released the new version of diffpy.structure, we should add this back to the requirements and run the test on CI. This partially fixes #152 but we need to remove the skip for diffpy.srreal so that we could close this issue.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.63%. Comparing base (e2b5c46) to head (43d83a1).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
+ Coverage   69.36%   71.63%   +2.26%     
==========================================
  Files          25       25              
  Lines        3457     3437      -20     
==========================================
+ Hits         2398     2462      +64     
+ Misses       1059      975      -84     

Files with missing lines	Coverage Δ
tests/conftest.py	88.40% <ø> (-3.09%)	⬇️
tests/test_diffpyparset.py	98.79% <100.00%> (+82.12%)	⬆️
tests/test_pdf.py	47.82% <100.00%> (+4.52%)	⬆️

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