BUG: Fix interval_range when start/periods or end/periods are specified with float start/end
- [X] closes #21161
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

Should be good for 0.23.1; the code changes only hit the `interval_range` function, which is outside the `IntervalIndex` class, so should be independent of any enhancements related to `IntervalIndex` that we might want to push to 0.24.0.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21162?src=pr&el=h1) Report
> Merging [#21162](https://codecov.io/gh/pandas-dev/pandas/pull/21162?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cd04471023d7b02dfcc168e5bdfcf1d7f960e8aa?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21162/graphs/tree.svg?height=150&token=eZ4WkYLtcO&src=pr&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/21162?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21162   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files         153      153           
  Lines       49505    49505           
=======================================
  Hits        45466    45466           
  Misses       4039     4039
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <100%> (ø)` | :arrow_up: |
| #single | `41.88% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21162?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/21162/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `93.14% <100%> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21162?src=pr&el=footer). Last update [cd04471...6520eb8](https://codecov.io/gh/pandas-dev/pandas/pull/21162?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. looks like a conflict in whatsother. merge on fix. (note ok to add some blank lines in each of the bug fix sections to avoid future conflicts)