add Python 3.8 to the matrix and use custom script to install it

Author: skvark

appveyor.yml

@@ -46,6 +46,14 @@ environment:
       ENABLE_CONTRIB: 0
       ENABLE_HEADLESS: 0
 
+    - PYTHON: "C:/Python38"
+      ENABLE_CONTRIB: 0
+      ENABLE_HEADLESS: 0
+
+    - PYTHON: "C:/Python38-x64"
+      ENABLE_CONTRIB: 0
+      ENABLE_HEADLESS: 0
+
     - PYTHON: "C:/Python27"
       ENABLE_CONTRIB: 1
       ENABLE_HEADLESS: 0
@@ -86,6 +94,14 @@ environment:
       ENABLE_CONTRIB: 1
       ENABLE_HEADLESS: 0
 
+    - PYTHON: "C:/Python38"
+      ENABLE_CONTRIB: 1
+      ENABLE_HEADLESS: 0
+
+    - PYTHON: "C:/Python38-x64"
+      ENABLE_CONTRIB: 1
+      ENABLE_HEADLESS: 0
+
     - PYTHON: "C:/Python27"
       ENABLE_CONTRIB: 0
       ENABLE_HEADLESS: 1
@@ -126,6 +142,14 @@ environment:
       ENABLE_CONTRIB: 0
       ENABLE_HEADLESS: 1
 
+    - PYTHON: "C:/Python38"
+      ENABLE_CONTRIB: 0
+      ENABLE_HEADLESS: 1
+
+    - PYTHON: "C:/Python38-x64"
+      ENABLE_CONTRIB: 0
+      ENABLE_HEADLESS: 1
+
     - PYTHON: "C:/Python27"
       ENABLE_CONTRIB: 1
       ENABLE_HEADLESS: 1
@@ -166,9 +190,24 @@ environment:
       ENABLE_CONTRIB: 1
       ENABLE_HEADLESS: 1
 
+    - PYTHON: "C:/Python38"
+      ENABLE_CONTRIB: 1
+      ENABLE_HEADLESS: 1
+
+    - PYTHON: "C:/Python38-x64"
+      ENABLE_CONTRIB: 1
+      ENABLE_HEADLESS: 1
+
 matrix:
     fast_finish: true
 
+install:
+  - ps: |
+      if (-not (Test-Path $env:PYTHON)) {
+        curl -o install_python.ps1 https://raw.githubusercontent.com/matthew-brett/multibuild/11a389d78892cf90addac8f69433d5e22bfa422a/install_python.ps1
+        .\install_python.ps1
+      }
+
 build_script:
 - cmd: |
     "%PYTHON%/python.exe" -m pip install --upgrade pip