Remove PY3 compatibility shim

[blink1073]

Summary

• Remove PY3 = sys.version_info[0] == 3 from _metakernel.py and magic.py
• Drop the if not PY3: kwargs["shell"] = None branch in MetaKernel.__init__
• Simplify both inspect.getfullargspec(func) if PY3 else inspect.getargspec(func) calls to just inspect.getfullargspec(func)

Since only Python 3 is supported, these shims are dead code.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.64%. Comparing base (a524854) to head (64a7d49).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #334      +/-   ##
==========================================
+ Coverage   86.59%   86.64%   +0.05%     
==========================================
  Files          51       51              
  Lines        2797     2793       -4     
  Branches      388      387       -1     
==========================================
- Hits         2422     2420       -2     
+ Misses        257      256       -1     
+ Partials      118      117       -1     

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