fix(py): allow conditional cases to be defined out of order

[ss2165]

Closes #1596

Was tempted to just change the type of cases since who would be using it anyway but I've tried to be good and deprecate instead

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.71%. Comparing base (c5b597d) to head (a15a89e).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-py/src/hugr/build/cond_loop.py	80.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1599      +/-   ##
==========================================
- Coverage   85.72%   85.71%   -0.01%     
==========================================
  Files         136      136              
  Lines       24783    24793      +10     
  Branches    21719    21719              
==========================================
+ Hits        21244    21251       +7     
- Misses       2437     2440       +3     
  Partials     1102     1102              

Flag	Coverage Δ
python	92.61% <80.00%> (-0.08%)	⬇️

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