Ensure templatize_constraint returns an expression

[jsiirola]

Fixes # .

Summary/Motivation:

@codykarcher reported two errors with templatize_constraint: first, the templatized expression was not guaranteed to be an expression (it could contain a tuple), and the other was a missing import. This PR resolves both of those issues.

Changes proposed in this PR:

• Ensure that the first return value from templatize_constraint is always an expression
• Add tests

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (1764336) 88.06% compared to head (ecbd709) 88.07%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2983      +/-   ##
==========================================
+ Coverage   88.06%   88.07%   +0.01%     
==========================================
  Files         769      769              
  Lines       89454    89476      +22     
==========================================
+ Hits        78780    78810      +30     
+ Misses      10674    10666       -8     

Flag	Coverage Δ
linux	85.18% <100.00%> (-0.01%)	⬇️
osx	74.98% <100.00%> (+<0.01%)	⬆️
other	85.36% <100.00%> (+<0.01%)	⬆️
win	82.42% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
pyomo/core/expr/relational_expr.py	89.50% <100.00%> (+0.23%)	⬆️
pyomo/core/expr/template_expr.py	87.76% <100.00%> (+0.07%)	⬆️

... and 15 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.