[python][ci] start to support Python 3.8 (microsoft#2713)

* start to support Python 3.8

* update configs

* hotfix

.travis.yml

@@ -12,7 +12,7 @@ osx_image: xcode11.3
 
 env:
   global:  # default values
-    - PYTHON_VERSION=3.7
+    - PYTHON_VERSION=3.8
   matrix:
     - TASK=regular PYTHON_VERSION=3.6
     - TASK=sdist PYTHON_VERSION=2.7
@@ -21,7 +21,7 @@ env:
     - TASK=lint
     - TASK=check-docs
     - TASK=mpi METHOD=source
-    - TASK=mpi METHOD=pip
+    - TASK=mpi METHOD=pip PYTHON_VERSION=3.7
     - TASK=gpu METHOD=source PYTHON_VERSION=3.5
     - TASK=gpu METHOD=pip PYTHON_VERSION=3.6
     - TASK=r-package

.vsts-ci.yml

@@ -6,7 +6,7 @@ trigger:
     include:
     - v*
 variables:
-  PYTHON_VERSION: 3.7
+  PYTHON_VERSION: 3.8
   CONDA_ENV: test-env
 resources:
   containers:
@@ -31,7 +31,7 @@ jobs:
         PYTHON_VERSION: 3.5
       bdist:
         TASK: bdist
-        PYTHON_VERSION: 3.6
+        PYTHON_VERSION: 3.7
       inference:
         TASK: if-else
       mpi_source:
@@ -80,7 +80,7 @@ jobs:
     matrix:
       regular:
         TASK: regular
-        PYTHON_VERSION: 3.6
+        PYTHON_VERSION: 3.7
       sdist:
         TASK: sdist
         PYTHON_VERSION: 3.5
@@ -121,13 +121,12 @@ jobs:
     matrix:
       regular:
         TASK: regular
-        PYTHON_VERSION: 3.7
+        PYTHON_VERSION: 3.6
       sdist:
         TASK: sdist
         PYTHON_VERSION: 2.7
       bdist:
         TASK: bdist
-        PYTHON_VERSION: 3.5
   steps:
   - powershell: |
       Write-Host "##vso[task.prependpath]$env:CONDA\Scripts"

python-package/setup.py

@@ -304,4 +304,5 @@ def run(self):
                        'Programming Language :: Python :: 3.5',
                        'Programming Language :: Python :: 3.6',
                        'Programming Language :: Python :: 3.7',
+                       'Programming Language :: Python :: 3.8',
                        'Topic :: Scientific/Engineering :: Artificial Intelligence'])