add python 3.8.0rc1 to windows possibilities

.appveyor.yml

@@ -80,3 +80,5 @@ test_script:
     - ps: .\install_python.ps1
     - set PYTHON=C:\Python37-x64
     - ps: .\install_python.ps1
+    - set PYTHON=C:\Python38-x64
+    - ps: .\install_python.ps1

.gitignore

@@ -1,2 +1,3 @@
 downloads/
 *.orig
+*.swp

install_python.ps1

@@ -21,7 +21,14 @@ if ($env:PYTHON -eq "C:\Python${req_nodot}-x64") {
 $py_url = "https://www.python.org/ftp/python"
 Write-Host "Installing Python ${req_ver}$exe_suffix..." -ForegroundColor Cyan
 $exePath = "$env:TEMP\python-${req_ver}${exe_suffix}.exe"
-$downloadFile = "$py_url/${req_ver}/python-${req_ver}${exe_suffix}.exe"
+
+if ($req_nodot -eq "38") {
+    $downloadFile = "$py_url/${req_ver}/python-${req_ver}rc1${exe_suffix}.exe"
+}
+else {
+    $downloadFile = "$py_url/${req_ver}/python-${req_ver}${exe_suffix}.exe"
+}
+
 Write-Host "Downloading $downloadFile..."
 (New-Object Net.WebClient).DownloadFile($downloadFile, $exePath)
 Write-Host "Installing..."