CI: move 2.7_SLOW -> 3.6_SLOW
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20559?src=pr&el=h1) Report
> Merging [#20559](https://codecov.io/gh/pandas-dev/pandas/pull/20559?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0a003651a67854b9776c46469a18e20ffd61f141?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20559/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20559?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20559      +/-   ##
==========================================
- Coverage   91.83%   91.82%   -0.02%     
==========================================
  Files         152      152              
  Lines       49260    49268       +8     
==========================================
- Hits        45240    45238       -2     
- Misses       4020     4030      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.2% <ø> (-0.02%)` | :arrow_down: |
| #single | `41.89% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20559?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20559/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20559/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.19% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/20559/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.88% <0%> (+0.02%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20559/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.73% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20559?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/20559?src=pr&el=footer). Last update [0a00365...3e684dc](https://codecov.io/gh/pandas-dev/pandas/pull/20559?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
