EHN: Implement method argument for DataFrame.replace
- [x] closes #19632
- [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/19894?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@d14fae8`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19894/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19894?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19894   +/-   ##
=========================================
  Coverage          ?   91.69%           
=========================================
  Files             ?      150           
  Lines             ?    49051           
  Branches          ?        0           
=========================================
  Hits              ?    44977           
  Misses            ?     4074           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.07% <100%> (?)` | |
| #single | `41.79% <0%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19894?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/19894/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.84% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19894?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/19894?src=pr&el=footer). Last update [d14fae8...5d8eae5](https://codecov.io/gh/pandas-dev/pandas/pull/19894?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 very minor comment. ping when pushed. also there *might* be a duplicate issue of this out there, if you find one lmk. @jreback I don't think your comment was posted.

I had a look for a duplicate issue but couldn't find any. @jreback it’s green now, thanks! hmm for some reason this is not triggering a re-run, looks like travis is partially degraded atm thanks!