pip cache requires pip 20.1

.travis.yml

@@ -266,7 +266,7 @@ jobs:
       env: DOCKER_IMAGE=quay.io/pypa/manylinux2010_x86_64
       install: docker pull $DOCKER_IMAGE
       script: bash scripts/releases/make-manylinux
-      before_script: pip install -U twine
+      before_script: python -mpip install -U pip twine
 
     - stage: test
       name: 32-bit manylinux wheels (all Pythons)
@@ -275,7 +275,7 @@ jobs:
       env: DOCKER_IMAGE=quay.io/pypa/manylinux2010_i686 PRE_CMD=linux32
       install: docker pull $DOCKER_IMAGE
       script: bash scripts/releases/make-manylinux
-      before_script: pip install -U twine
+      before_script: python -mpip install -U pip twine
 
     # Lint the code. Because this is a separate job, even if it fails fast
     # the tests will still run. Put it at the top for fast feedback.

docs/changes/1628.feature

 Add support for Python 3.9.
 
-No binary wheels are available yet, however, and one must use a
-greenlet built from current git master.
+No binary wheels are available yet, however.

scripts/releases/make-manylinux

@@ -88,6 +88,10 @@ fi
 
 # Mount the current directory as /gevent
 # Mount the pip cache directory as /cache
+# `pip cache` requires pip 20.1
+echo Setting up caching
+python --version
+python -mpip --version
 LCACHE="$(dirname `python -mpip cache dir`)"
 echo Sharing pip cache at $LCACHE
 echo Sharing ccache dir at $HOME/.ccache