Run AStyle if it is installed

Also try to install it on AppVeyor

appveyor.yml

@@ -12,7 +12,12 @@ environment:
 
 init:
   - call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars%BITS%.bat"
-  - set PATH="C:\\Python37";%PATH%
+  - set PATH="C:\\Python37";"C:\\Python37\Scripts";"C:\\tmp\\tools";%PATH%
+  - mkdir "C:\tmp\tools"
+    # Download AStyle 3.1: first enable strong crypto in Invoke-WebRequest
+  - ps: Set-ItemProperty -Path "HKLM:\SOFTWARE\Wow6432Node\Microsoft\.NetFramework\v4.0.30319" -Name 'SchUseStrongCrypto' -Value '1' -Type DWord
+  - ps: Set-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\.NetFramework\v4.0.30319" -Name "SchUseStrongCrypto" -Value '1' -Type DWord
+  - ps: Invoke-WebRequest -OutFile "C:\tmp\tools\AStyle.exe" "https://rded.nl/pqclean/AStyle.exe"
 
 build_script:
   - python -m pip install -r requirements.txt

test/test_format.py

@@ -1,8 +1,8 @@
-import os
-from glob import glob
+import shutil
+import unittest
 
 import pqclean
-from helpers import run_subprocess, skip_windows
+from helpers import run_subprocess
 
 
 def test_formatting():
@@ -11,11 +11,11 @@ def test_formatting():
             yield check_format, implementation
 
 
-@skip_windows(message="This test needs to be amended to work with Windows "
-                      "installations of astyle")
 def check_format(implementation: pqclean.Implementation):
-    cfiles = glob(os.path.join(implementation.path(), '*.c'))
-    hfiles = glob(os.path.join(implementation.path(), '*.h'))
+    if shutil.which('astyle') is None:
+        raise unittest.SkipTest("AStyle is not installed")
+    cfiles = implementation.cfiles()
+    hfiles = implementation.hfiles()
     run_subprocess(['astyle',
                     '--dry-run',
                     '--options=../.astylerc',