BUG: make dense ranks results scale to 100 percent
- [x] closes #20731
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21203?src=pr&el=h1) Report
> Merging [#21203](https://codecov.io/gh/pandas-dev/pandas/pull/21203?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5348e06c4e9e8a03cbd0011483d2dd087e850940?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21203/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/21203?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21203   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files         153      153           
  Lines       49543    49543           
=======================================
  Hits        45504    45504           
  Misses       4039     4039
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.24% <ø> (ø)` | :arrow_up: |
| #single | `41.87% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21203?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21203?src=pr&el=footer). Last update [5348e06...c5b2756](https://codecov.io/gh/pandas-dev/pandas/pull/21203?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I guess this should be in 0.24 as its a numeric change. any takers for 0.23.1? I think its minor enough to be in 0.23.1 but don't have that strong a preference either way Thanks @peterpanmj !