Fix floattype() for non-concrete types while preserving the depwarn

[kimikage]

This is a variant of PR #354 with deprecated fallback method retained.

While it would be hypocritical not to call this a breaking change, it does allow us to avoid a direct violation of SemVer.
In other words, I plan to release this in the v0.9 series.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.74%. Comparing base (d62abf3) to head (bd93684).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #362      +/-   ##
==========================================
- Coverage   96.75%   96.74%   -0.01%     
==========================================
  Files           7        7              
  Lines         800      799       -1     
==========================================
- Hits          774      773       -1     
  Misses         26       26              

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.