[ci] try newer pip in wheel check

.ci/gitlab/ci.yml

@@ -137,6 +137,8 @@ stages:
       - devpi use http://pymor__devpi:3141/root/public --set-cfg
       - devpi login root --password ''
       - devpi upload --from-dir --formats=* ./dist/*.whl
+      - python3 -m pip install pip~=21.0
+      - python3 -m pip remove -y pymor || true
     # the docker service adressing fails on other runners
     tags: [mike]
 

.ci/gitlab/template.ci.py

@@ -147,6 +147,8 @@ rules:
       - devpi use http://pymor__devpi:3141/root/public --set-cfg
       - devpi login root --password ''
       - devpi upload --from-dir --formats=* ./dist/*.whl
+      - python3 -m pip install pip~=21.0
+      - python3 -m pip remove -y pymor || true
     # the docker service adressing fails on other runners
     tags: [mike]
 

README.md

@@ -71,7 +71,7 @@ command:
     pip install --upgrade pip  # make sure that pip is reasonably new
     pip install pymor[full]
 
-(Please note that pip must be at least version 19.0)
+(Please note that pip versions prior to 21.1 might have problems resolving all dependencies)
 
 This will install the latest release of pyMOR on your system with most optional
 dependencies.