Add Mergify config

[hugovk]

Fixes #4900.

Changes proposed in this pull request:

• Approval, label and Travis CI rules like in Like Add Mergify config docker-images#84 and Add Mergify config pillow-wheels#161
• AppVeyor rule is similar
• GHA: https://doc.mergify.io/conditions.html#github-actions

GitHub Actions works slightly differently. To match a status check when using GitHub Action, only the job name is used.

In the example above, it would be A job to say hello:

conditions:
  - status-success=A job to say hello

So it's a bit more maintenance for GHA. We could rename the jobs so they don't include Python versions etc., but it's useful to see which one fails at a glance. Let's see how it goes and iterate if necessary.

[nulano]

It might be possible to use dependant jobs in GHA to simplify this config a bit. Each workflow set (test.yml, test-docker.yml, test-windows.yml) can have another dummy job added that depends on the success of the rest of the set named something like Test Windows / All Successful which could allow this config to be just three GHA lines.

I'm not sure whether there is anything else that could be shown in these dummy jobs such as perhaps a warnings summary, but it might be worth adding just to simplify this config. OTOH the set of jobs doesn't change often, so this might be fine as is.

[hugovk]

Interesting idea!

Would they still need enumerating, something like this?

jobs:
  build:
    ...
  success:
    needs: [
      MSYS2 MinGW 32-bit,
      MSYS2 MinGW 64-bit,
      Python 3.6 x64,
      Python 3.6 x86,
      Python 3.7 x64,
      Python 3.7 x86,
      Python 3.8 x64,
      Python 3.8 x86,
      Python 3.9-dev x64,
      Python 3.9-dev x86,
      Python pypy3 x86,
    ]

Or would it be possible to use a matrix?

[nulano]

It only needs to list build:

jobs:
  build:
    ...
  success:
    needs: build
    ...

Take a look at an example I made: test.yml, test-windows.yml

[hugovk]

That looks good. Please could you make a test PR against my fork? I've put this config there:

hugovk@5b6c6fc

[hugovk]

The tests worked (hugovk#60, hugovk#61), so updated this PR!