Added missing import in pulsar_binary.py

[abhisrkckl]

Fixes bug #1356

Do I need to create a test for this?

[codecov]

Codecov Report

Merging #1360 (84f29ff) into master (bfa227e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1360   +/-   ##
=======================================
  Coverage   61.29%   61.29%           
=======================================
  Files          90       90           
  Lines       20351    20351           
  Branches     3646     3646           
=======================================
  Hits        12475    12475           
  Misses       7089     7089           
  Partials      787      787           

Impacted Files	Coverage Δ
src/pint/models/pulsar_binary.py	76.24% <100.00%> (ø)

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