BUG: Fix marker for high memory
Added strict flag to catch this in the future

xref https://github.com/pandas-dev/pandas/pull/18427#discussion_r153277727
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18526?src=pr&el=h1) Report
> Merging [#18526](https://codecov.io/gh/pandas-dev/pandas/pull/18526?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/34b036c6096cf7fdd9d17e9af4ed8cfed9525dcf?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18526/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18526?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18526      +/-   ##
==========================================
- Coverage   91.35%   91.33%   -0.03%     
==========================================
  Files         164      164              
  Lines       49819    49801      -18     
==========================================
- Hits        45512    45485      -27     
- Misses       4307     4316       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.13% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.81% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18526?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18526?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/18526?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.9% <0%> (-0.05%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18526?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/18526?src=pr&el=footer). Last update [34b036c...63a338b](https://codecov.io/gh/pandas-dev/pandas/pull/18526?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @TomAugspurger 