Fix version check for system SUNDIALS on Windows / alpha version bump

[speth]

Changes proposed in this pull request

• SCons's, conf.TryRun apparently doesn't use Python's universal newlines, so we need to explicitly include \r in the set of characters to strip. (Fixes regression introduced in Remove CT_SUNDIALS_VERSION constant in favor of SUNDIALS-defined constants #1746).
• Fix a couple of warnings about size_t/int conversions
• Bump version to 3.1.0a3

If applicable, provide an example illustrating new features this pull request is introducing

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.13%. Comparing base (520267d) to head (ec32f7b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1755   +/-   ##
=======================================
  Coverage   73.13%   73.13%           
=======================================
  Files         381      381           
  Lines       54134    54134           
  Branches     9224     9224           
=======================================
  Hits        39590    39590           
  Misses      11587    11587           
  Partials     2957     2957           

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[ischoegl]

Thanks! LGTM.