Fix: use duck-typing in Pulsar() to determine whether we have a libstempo or PINT object

[vhaasteren]

When packages write tests in their test suite, sometimes Mock objects are used instead of libstempo/PINT pulsars. Or perhaps there are other reasons to not pass a libstempo/PINT object. Then Enterprise could still make a PintPulsar or a Tempo2Pulsar object if we were to use duck-typing. See #424

This PR updates it. No functionality is changed. Just a change from 'isinstance' to a duck-typing indicator function

[codecov]

Codecov Report

❌ Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.59%. Comparing base (be58281) to head (742ee45).
⚠️ Report is 10 commits behind head on dev.

Files with missing lines	Patch %	Lines
enterprise/pulsar.py	63.63%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #425      +/-   ##
==========================================
+ Coverage   72.52%   72.59%   +0.07%     
==========================================
  Files          13       13              
  Lines        3472     3478       +6     
==========================================
+ Hits         2518     2525       +7     
+ Misses        954      953       -1     

Files with missing lines	Coverage Δ
enterprise/pulsar.py	29.80% <63.63%> (+1.03%)	⬆️

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