DEPR: Deprecate tupleize_cols in Index constructor
Title is self-explanatory.

xref #17060.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=h1) Report
> Merging [#17899](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5bf7f9a4ff6968fd444fa7098f8dc95586591994?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17899/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17899      +/-   ##
==========================================
- Coverage   91.23%   91.22%   -0.02%     
==========================================
  Files         163      163              
  Lines       50105    50111       +6     
==========================================
- Hits        45715    45712       -3     
- Misses       4390     4399       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <100%> (ø)` | :arrow_up: |
| #single | `40.32% <85.71%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `95.14% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.43% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17899?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/17899?src=pr&el=footer). Last update [5bf7f9a...613499c](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=h1) Report
> Merging [#17899](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5bf7f9a4ff6968fd444fa7098f8dc95586591994?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17899/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17899      +/-   ##
==========================================
- Coverage   91.23%   91.22%   -0.02%     
==========================================
  Files         163      163              
  Lines       50105    50111       +6     
==========================================
- Hits        45715    45712       -3     
- Misses       4390     4399       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <100%> (ø)` | :arrow_up: |
| #single | `40.32% <85.71%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.43% <100%> (+0.01%)` | :arrow_up: |
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `95.14% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17899?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/17899?src=pr&el=footer). Last update [5bf7f9a...613499c](https://codecov.io/gh/pandas-dev/pandas/pull/17899?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I don't think we can deprecate this. When we implement the "`dtype=objcet` disables all inference" (and thus preserves the tuples as tuples instead of returning MultiIndex) (https://github.com/pandas-dev/pandas/issues/17246), we have an alternative way to do this, and we *might* consider deprecating `tupleize_cols`. But https://github.com/pandas-dev/pandas/issues/17246 hasn't been solved yet. This is also what Tom mentioned in https://github.com/pandas-dev/pandas/pull/17060#issuecomment-323775252

> > Including the Index constructor one (just to confirm)?

> I think so, with the idea the Index([('a', 1)]) -> MultiIndex, while Index([('a', 1)], dtype=object) returns a regular Index with tuples for values. xref #17246

The main thing that would not be possible with `dtype=object` is to "infer the dtype, but never return MI (so don't infer tuples to MI)", which is possible now with `tupleize_cols=False` Ah, right...thanks for reminding me about that.  I guess the deprecation does seem premature in light of the ongoing discussion in #17246.

I'll close the PR but can always revisit once it gets resolved. > the ongoing discussion in #17246.

I *think* we agree on the idea, it just needs somebody doing the work.