appveyor: Upgrade pip with 'python -m pip'

This reportedly works around https://github.com/pypa/pip/issues/5240

diff --git a/appveyor.yml b/appveyor.yml
index 620c338b993978dd3f456d797ce9206fabacf880..72f9d881d344beae1811089e82cc198ccec21a86 100644 (file)
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -48,9 +48,9 @@ install:
 
   # Upgrade to the latest version of pip to avoid it displaying warnings
   # about it being out of date.
-  - "pip install --disable-pip-version-check --user --upgrade pip"
+  - "python -m pip install --disable-pip-version-check --user --upgrade pip"
 
-  - "pip install tox wheel"
+  - "python -m pip install tox wheel"
 
 build: false  # Not a C# project, build stuff at the test step instead.