[AIRFLOW-6069] Python host version in travis is set to 3.6 always (#6666)

This will make the scripts more "stable" - no problems with
features missing in 3.5 for host scripts.

Python version for all tests in container is controlled via
PYTHON_VERSION variable.

.travis.yml

@@ -33,7 +33,6 @@ stages:
 jobs:
   include:
     - name: "Static checks"
-      python: "3.6"
       stage: pre-test
       script: ./scripts/ci/ci_run_all_static_tests.sh
       env: >-
@@ -42,7 +41,6 @@ jobs:
     - name: "Build documentation"
       env: >-
         PYTHON_VERSION=3.6
-      python: "3.6"
       stage: pre-test
       script: ./scripts/ci/ci_docs.sh
     - name: "Tests postgres python 3.6"
@@ -50,21 +48,18 @@ jobs:
         BACKEND=postgres
         ENV=docker
         PYTHON_VERSION=3.6
-      python: "3.6"
       stage: test
     - name: "Tests sqlite python 3.6"
       env:
         BACKEND=sqlite
         ENV=docker
         PYTHON_VERSION=3.6
-      python: "3.6"
       stage: test
     - name: "Tests mysql python 3.7"
       env:
         BACKEND=mysql
         ENV=docker
         PYTHON_VERSION=3.7
-      python: "3.7"
       stage: test
     - name: "Tests postgres kubernetes python 3.6 (persistent)"
       env: >-
@@ -73,7 +68,6 @@ jobs:
         KUBERNETES_VERSION=v1.15.0
         KUBERNETES_MODE=persistent_mode
         PYTHON_VERSION=3.6
-      python: "3.6"
       stage: test
       script: travis_wait 30 "./scripts/ci/ci_run_airflow_testing.sh"
     - name: "Tests postgres kubernetes python 3.6 (git)"
@@ -83,7 +77,6 @@ jobs:
         KUBERNETES_VERSION=v1.15.0
         KUBERNETES_MODE=git_mode
         PYTHON_VERSION=3.6
-      python: "3.6"
       stage: test
       script: travis_wait 30 "./scripts/ci/ci_run_airflow_testing.sh"
   services: