Tweak appveyor and instructions for #1412. Fixes #1412

appveyor.yml

@@ -128,7 +128,7 @@ install:
 
   # Upgrade to the latest version of pip to avoid it displaying warnings
   # about it being out of date.
-  - "%CMD_IN_ENV% %PYEXE% -m pip install --disable-pip-version-check --user --upgrade pip"
+  - "%CMD_IN_ENV% %PYEXE% -m pip install --disable-pip-version-check --user --upgrade pip<19.1"
 
   - ps: "if(Test-Path(\"${env:PYTHON}\\bin\")) {ls ${env:PYTHON}\\bin;}"
   - ps: "if(Test-Path(\"${env:PYTHON}\\Scripts\")) {ls ${env:PYTHON}\\Scripts;}"
@@ -139,6 +139,7 @@ cache:
 
 build_script:
   # Build the compiled extension
+  # pip 19.1 (exactly) won't work; but 19.1.1 should with --no-use-pep517
   - if not "%GWHEEL_ONLY%"=="true" %PYEXE% -m pip install -U --upgrade-strategy=eager -r dev-requirements.txt
 
 test_script:

docs/install.rst

@@ -163,6 +163,12 @@ To hack on gevent (using a virtualenv)::
    prerequisites (CFFI, Cython), but having a working C compiler that
    can create Python extensions is up to you.
 
+.. warning::
+
+   This pip command does not work with pip 19.1. Either use pip 19.0
+   or below, or use pip 19.1.1 with ``--no-use-pep517``. See `issue
+   1412 <https://github.com/gevent/gevent/issues/1412>`_.
+
 
 Running Tests
 -------------