enable pip cache in appveyor build

.appveyor.yml

@@ -14,6 +14,9 @@ install:
   - C:\Python%PYTHON%\python.exe -m pip install -U pip setuptools
   - C:\Python%PYTHON%\python.exe -m pip install .[test,websupport]
 
+cache:
+  - '%LOCALAPPDATA%\pip\Cache'
+
 # No automatic build, just run python tests
 build: off
 
@@ -39,3 +42,17 @@ test_script:
 
 after_test:
   - ps: (New-Object System.Net.WebClient).UploadFile("https://ci.appveyor.com/api/testresults/junit/$($env:APPVEYOR_JOB_ID)", (Resolve-Path (Join-Path tests .junit.xml)))
+
+after_build:
+  # Remove old or huge cache files to hopefully not exceed the 1GB cache limit.
+  #
+  # If the cache limit is reached, the cache will not be updated (of not even
+  # created in the first run). So this is a trade of between keeping the cache
+  # current and having a cache at all.
+  # NB: This is done only `on_success` since the cache in uploaded only on
+  # success anyway.
+  - C:\cygwin\bin\find "%LOCALAPPDATA%\pip" -type f -mtime +360 -delete
+  - C:\cygwin\bin\find "%LOCALAPPDATA%\pip" -type f -size +10M -delete
+  - C:\cygwin\bin\find "%LOCALAPPDATA%\pip" -empty -delete
+  # Show size of cache
+  - C:\cygwin\bin\du -hs "%LOCALAPPDATA%\pip\Cache"