ENH: Let initialisation from dicts use insertion order for python >= 3.6 (part I)
- [x] xref #19018
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This PR prepares pandas to allow initialisation using insertion order for python>=3.6. E.g. ``Series({'B': 1, 'A': 2})`` will then retain order and not convert to ``Series({'A': 2, 'B': 1})``.

Letting dataframe/series initialisation from dicts preserve order for python >=3.6 is a relative simple change, just change a line each in core/frame.py and core/series.py. However, this makes some 36 tests fail, as the tests have been set up to depend on conversion to alphabetical order.

This PR makes all tests in ``pandas/tests/frame`` pass both if dict initialisation uses insertion order and alphabetical order. No test functionality is changed in this PR.

After this PR I will do 1 or 2 more similar PRs, so all tests pass independent of dict initialisation rule, and then core/frame.py and core/series.py will be changed to use insertion order for python>=3.6.
seems fine. small comments. ping on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19830?src=pr&el=h1) Report
> Merging [#19830](https://codecov.io/gh/pandas-dev/pandas/pull/19830?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8768876a51a3dfa0625bab6925caca8b3e9fcceb?src=pr&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19830/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19830?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19830      +/-   ##
==========================================
+ Coverage   91.58%   91.61%   +0.03%     
==========================================
  Files         150      150              
  Lines       48908    48892      -16     
==========================================
+ Hits        44792    44793       +1     
+ Misses       4116     4099      -17
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.99% <ø> (+0.03%)` | :arrow_up: |
| #single | `41.79% <ø> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19830?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/tile.py](https://codecov.io/gh/pandas-dev/pandas/pull/19830/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS90aWxlLnB5) | `90.25% <0%> (-2.69%)` | :arrow_down: |
| [pandas/util/\_validators.py](https://codecov.io/gh/pandas-dev/pandas/pull/19830/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX3ZhbGlkYXRvcnMucHk=) | `96.34% <0%> (-0.47%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/19830/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.32% <0%> (-0.25%)` | :arrow_down: |
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/19830/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `95.65% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/19830/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `90.56% <0%> (-0.07%)` | :arrow_down: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19830/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `96.79% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/19830/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `94.9% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19830/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.23% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/19830/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.94% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19830/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.45% <0%> (+0.03%)` | :arrow_up: |
| ... and [8 more](https://codecov.io/gh/pandas-dev/pandas/pull/19830/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19830?src=pr&el=footer). Last update [8768876...d677f94](https://codecov.io/gh/pandas-dev/pandas/pull/19830?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Green. lgtm ping on green thanks