main/py3-yaml: add dependency on python3

Before this change:

    $ dabuild sh
    
    $ sudo apk add --no-progress --quiet py3-yaml
    $ python3 -c 'import yaml; print(yaml.__version__)'
    sh: python3: not found

After this change:

    $ dabuild sh
    
    $ sudo apk add --no-progress --quiet py3-yaml
    $ python3 -c 'import yaml; print(yaml.__version__)'
    5.3.1
7 jobs for !9022 with py3-yaml in 15 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Lint
failed #138500
docker-alpine x86_64 allowed to fail
lint

00:00:06

 
  Build
failed #138505
docker-alpine ci-build aarch64
build-aarch64

00:00:06

failed #138506
docker-alpine ci-build armv7
build-armv7

00:00:07

failed #138504
docker-alpine ci-build ppc64le
build-ppc64le

00:00:08

failed #138503
docker-alpine ci-build s390x
build-s390x

00:00:05

failed #138502
docker-alpine ci-build x86
build-x86

00:00:06

failed #138501
docker-alpine ci-build x86_64
build-x86_64

00:00:06

 
Name Stage Failure
failed
lint Lint
No job log
failed
build-s390x Build
No job log
failed
build-aarch64 Build
No job log
failed
build-x86 Build
No job log
failed
build-armv7 Build
No job log
failed
build-ppc64le Build
No job log
failed
build-x86_64 Build
No job log