CI: Fix yaml/yml inconsistency
The ci/windows.yml and ci/windows-py27.yml files reference the environment files with ".yml".  This PR moves the ".yaml" files to the indicated location.

CI config is hard.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23001?src=pr&el=h1) Report
> Merging [#23001](https://codecov.io/gh/pandas-dev/pandas/pull/23001?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4976ee1c93bcc6327ca5e8714ad326c126f4151e?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23001/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23001?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23001      +/-   ##
==========================================
+ Coverage   92.19%   92.19%   +<.01%     
==========================================
  Files         169      169              
  Lines       50831    50835       +4     
==========================================
+ Hits        46864    46868       +4     
  Misses       3967     3967
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `42.35% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23001?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/23001/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `95.56% <0%> (+0.02%)` | :arrow_up: |
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/23001/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `92.04% <0%> (+0.06%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23001?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/23001?src=pr&el=footer). Last update [4976ee1...72a19ee](https://codecov.io/gh/pandas-dev/pandas/pull/23001?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @TomAugspurger looks like you're the local expert on this.  Any ideas? I guess azure is being clever and trying both? Regardless, standardizing on one seems good and  `.yaml` is more common.