disallow normalize=True with Tick classes
State: closed

The problem: allowing `Tick` objects with `normalize=True` causes addition to lose monotonicity/associativity

```
ts = pd.Timestamp.now()
tick = pd.offsets.Minute(n=4, normalize=True)
>>> ts
Timestamp('2018-06-11 10:50:14.419655')
>>> ts + tick
Timestamp('2018-06-11 00:00:00')
```

- [x] closes #21434
- [x] tests added/passed
- [x] passes flake8
- [x] whatsnew note

Labels:
name: Bug
name: Error Reporting
description: Incorrect or improved errors from pandas
name: Timedelta
description: Timedelta data type

Comments:
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21427?src=pr&el=h1) Report
> Merging [#21427](https://codecov.io/gh/pandas-dev/pandas/pull/21427?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/576d5c6b76e039a411a7cc4c0de29813e2de0149?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21427/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21427?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21427      +/-   ##
==========================================
+ Coverage    91.9%    91.9%   +<.01%     
==========================================
  Files         153      153              
  Lines       49606    49608       +2     
==========================================
+ Hits        45589    45591       +2     
  Misses       4017     4017
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <100%> (ø)` | :arrow_up: |
| #single | `41.89% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21427?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/21427/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.25% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21427?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/21427?src=pr&el=footer). Last update [576d5c6...d15d137](https://codecov.io/gh/pandas-dev/pandas/pull/21427?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

@jbrockmendel : If this is the correct way to fix (I'm not 100% sure @jreback ), then your PR is good to go.
> is there an associated issue?

@jreback : No, there isn't AFAIK.  I suggested to @jbrockmendel that one be opened only if there was disagreement about the fix.
Opened #21434, extends the example given in the OP and mentions that `Day` could be a special case (though I'd prefer it not)
@jbrockmendel you didn’t need to make an issue, but ok

can u make a sub section with an example in whatsnew

Just added a whatsnew section, not sure it adds much on top of the one-line note (also fleshed out)
thanks @jbrockmendel