fix DateOffset eq to depend on normalize attr
This has a bullet point in #18854 but doesn't appear to have its own issue.

Current:
```
now = pd.Timestamp.now()
off = pd.offsets.Week(n=1, normalize=False)
normed = pd.offsets.Week(n=1, normalize=True)

>>> now + off == now + normed
False
>>> off == normed  # PR changes this comparison to False
True
```

- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21404?src=pr&el=h1) Report
> Merging [#21404](https://codecov.io/gh/pandas-dev/pandas/pull/21404?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/576d5c6b76e039a411a7cc4c0de29813e2de0149?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/21404/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21404?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21404   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         153      153           
  Lines       49606    49606           
=======================================
  Hits        45589    45589           
  Misses       4017     4017
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <100%> (ø)` | :arrow_up: |
| #single | `41.89% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21404?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/21404/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.24% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21404?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/21404?src=pr&el=footer). Last update [576d5c6...b902cb1](https://codecov.io/gh/pandas-dev/pandas/pull/21404?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > This has a bullet point in #18854 but doesn't appear to have its own issue.

@jbrockmendel : How about this one here: https://github.com/pandas-dev/pandas/issues/17689?  It might be a little familiar to you. 😉  @jreback : Everything is green here. I merged something which now causes a conflict here. @jbrockmendel can you rebase. @gfyoung can merge on green. Thanks @jbrockmendel !