algebra cleanup in FY5253 and Easter offsets
Same types of cleanups you've gotten used to seeing elsewhere, now applied to FY5253 and Easter.  This should be orthogonal to others.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18350?src=pr&el=h1) Report
> Merging [#18350](https://codecov.io/gh/pandas-dev/pandas/pull/18350?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cfad581e9a6d35c7d05d2b1f34e4a19f7ee15cc6?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `90.69%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18350/graphs/tree.svg?src=pr&height=150&width=650&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18350?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18350      +/-   ##
==========================================
- Coverage   91.38%   91.37%   -0.02%     
==========================================
  Files         164      164              
  Lines       49790    49758      -32     
==========================================
- Hits        45501    45465      -36     
- Misses       4289     4293       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.17% <90.69%> (ø)` | :arrow_up: |
| #single | `39.51% <0%> (-0.05%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18350?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/18350?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.2% <90.69%> (+0.28%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18350?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18350?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18350?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/18350?src=pr&el=footer). Last update [cfad581...47fa1b9](https://codecov.io/gh/pandas-dev/pandas/pull/18350?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I think there's one more algebra/cleanup after this, then we're jumping into bugfixes. I doubt we have very many tests for this. @gfyoung can you have a look and see if the changes look ok. @jreback : I'll take a look later this evening.  If all looks good, can I just merge this? yep #14774 has a use case that hits one of the existing `assert False` statements (i.e. not one of the ones discussed above for coverage checks).  I'll take a look at that after this gets closed. @jbrockmendel : Merged and closed.  Thanks!