PERF: Use ccalendar.get_days_in_month over tslib.monthrange
asv looks like a wash, probably to be expected since these calls make up such a small part of the methods they are used in.

tslibs.resolution uses the stdlib calendar.monthrange instead of tslib.monthrange, so the perf bump should be bigger there.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21451?src=pr&el=h1) Report
> Merging [#21451](https://codecov.io/gh/pandas-dev/pandas/pull/21451?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ab668b0a56a9f2aee959bde787e9a0af4068d7a7?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/21451/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/21451?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21451   +/-   ##
=======================================
  Coverage   91.89%   91.89%           
=======================================
  Files         153      153           
  Lines       49600    49600           
=======================================
  Hits        45580    45580           
  Misses       4020     4020
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <100%> (ø)` | :arrow_up: |
| #single | `41.86% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21451?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/21451/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21451/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.8% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21451?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/21451?src=pr&el=footer). Last update [ab668b0...aecbee8](https://codecov.io/gh/pandas-dev/pandas/pull/21451?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jbrockmendel : Any `asv`-related information + `whatsnew` in `0.24.0` would be good here. this is fine, no whatsnew needed if just internal refactoring generally (unless its 'big') or has an api impact.