CLN: remove compat.iteritems
- [x] xref #25725
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Removes ``compat.iteritems``.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26079?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@190a69e`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `21.87%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26079/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26079?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #26079   +/-   ##
=========================================
  Coverage          ?   40.74%           
=========================================
  Files             ?      175           
  Lines             ?    52505           
  Branches          ?        0           
=========================================
  Hits              ?    21394           
  Misses            ?    31111           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #single | `40.74% <21.87%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26079?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/packers.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhY2tlcnMucHk=) | `14.53% <ø> (ø)` | |
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `69.73% <ø> (ø)` | |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `33.92% <0%> (ø)` | |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `48.26% <0%> (ø)` | |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `41.43% <0%> (ø)` | |
| [pandas/core/computation/align.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vYWxpZ24ucHk=) | `18.47% <0%> (ø)` | |
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `24.48% <0%> (ø)` | |
| [pandas/util/\_doctools.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RvY3Rvb2xzLnB5) | `12.87% <0%> (ø)` | |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `37.84% <0%> (ø)` | |
| [pandas/core/reshape/melt.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZWx0LnB5) | `13.33% <0%> (ø)` | |
| ... and [16 more](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26079?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/26079?src=pr&el=footer). Last update [190a69e...798fa45](https://codecov.io/gh/pandas-dev/pandas/pull/26079?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/26079?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@190a69e`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `89.23%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26079/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26079?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #26079   +/-   ##
=========================================
  Coverage          ?   91.94%           
=========================================
  Files             ?      175           
  Lines             ?    52430           
  Branches          ?        0           
=========================================
  Hits              ?    48207           
  Misses            ?     4223           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.5% <87.69%> (?)` | |
| #single | `40.73% <23.07%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26079?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/packers.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhY2tlcnMucHk=) | `88.08% <ø> (ø)` | |
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `93.1% <ø> (ø)` | |
| [pandas/util/\_doctools.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RvY3Rvb2xzLnB5) | `12% <0%> (ø)` | |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `83.76% <0%> (ø)` | |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `97.99% <100%> (ø)` | |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.61% <100%> (ø)` | |
| [pandas/io/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2h0bWwucHk=) | `92.64% <100%> (ø)` | |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.96% <100%> (ø)` | |
| [pandas/core/computation/align.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vYWxpZ24ucHk=) | `97.8% <100%> (ø)` | |
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `97.22% <100%> (ø)` | |
| ... and [16 more](https://codecov.io/gh/pandas-dev/pandas/pull/26079/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26079?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/26079?src=pr&el=footer). Last update [190a69e...1662318](https://codecov.io/gh/pandas-dev/pandas/pull/26079?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 The Travis error is a ResourceWarning: ``ResourceWarning: unclosed file <_io.TextIOWrapper name=0 mode='r' encoding='UTF-8'>``, All other stuff passes. Thanks @topper-123 !