Use bash on windows
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- towards https://github.com/pandas-dev/pandas/issues/26344

As per travis logs:
https://travis-ci.org/pandas-dev/pandas/jobs/552444580

```
ccache: /usr/bin/ccache
source deactivate
DeprecationWarning: 'source deactivate' is deprecated. Use 'conda deactivate'.
```
As of [conda 4.6 the preferred](https://www.anaconda.com/conda-4-6-release/) way of using an environment is to use `conda activate (env)`.
`source activate` will eventually be deprecated 

(only remaining place to do this is in `code_checks.yml` -> I will do this in a follow up for ease of review) 

* This make the steps in the `windows.yml` and `posix.yml` file very similar and we can potentially merge these.

* Use `run_tests.sh` on windows this will mean  we now tests with `pytest -n 2` and `pytest -n 1` on windows - now consistent with the linux builds

cc. @TomAugspurger @datapythonista 