CLN: rename lib.isscalar to lib.is_scalar
- [x] closes #18987
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

There's also a handful of methods which call `is_scalar` that are named `isscalar`, e.g.:
https://github.com/pandas-dev/pandas/blob/84335621ad0a0d83302a80b6911d3985c00b5cee/pandas/core/computation/ops.py#L101-L103
should these be changed as well?

Also does a CLN require an entry in whatsnew?


+1. This has bothered me for a while. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19000?src=pr&el=h1) Report
> Merging [#19000](https://codecov.io/gh/pandas-dev/pandas/pull/19000?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/84335621ad0a0d83302a80b6911d3985c00b5cee?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `75%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19000/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19000?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19000   +/-   ##
=======================================
  Coverage   91.58%   91.58%           
=======================================
  Files         150      150           
  Lines       48972    48972           
=======================================
  Hits        44851    44851           
  Misses       4121     4121
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.94% <75%> (ø)` | :arrow_up: |
| #single | `41.72% <25%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19000?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/19000/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.14% <0%> (ø)` | :arrow_up: |
| [pandas/core/dtypes/inference.py](https://codecov.io/gh/pandas-dev/pandas/pull/19000/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2luZmVyZW5jZS5weQ==) | `98.36% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/19000/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.04% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19000/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.45% <100%> (ø)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19000?src=pr&el=footer). Last update [8433562...b3e943d](https://codecov.io/gh/pandas-dev/pandas/pull/19000?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 you can change ``isscalar``  as you pointed above (and other instances as appropriate). making things more clear / consistent is the goal. don't need a whatsnew here as this is an internal change. lgtm. note that windows is currently failing, but ping when travis ci is green. thanks @gitavi 