Use environment markers to install cffi on windows.

Also try working around all the failed builds that pip 10 created: https://ci.appveyor.com/project/denik/gevent/build/1.0.1667/job/9jarsdxb6y0x2smd

CHANGES.rst

@@ -7,7 +7,8 @@
 1.3b2 (unreleased)
 ==================
 
-- Nothing changed yet.
+- On Windows, CFFI is now a dependency so that the libuv backend
+  really can be used by default.
 
 
 1.3b1 (2018-04-13)

appveyor.yml

@@ -125,8 +125,8 @@ install:
   # pip will build them from source using the MSVC compiler matching the
   # target Python version and architecture
   # Note that psutil won't build under PyPy on Windows.
-  - "%CMD_IN_ENV% pip install -U cython zope.interface zope.event"
-  - "%CMD_IN_ENV% pip install -U setuptools wheel greenlet cffi dnspython idna requests"
+  - "%CMD_IN_ENV% %PYEXE% -m pip install -U cython zope.interface zope.event"
+  - "%CMD_IN_ENV% %PYEXE% -m pip install -U setuptools wheel greenlet cffi dnspython idna requests"
 
   - ps:
       if ("${env:PYTHON_ID}" -ne "pypy") {

setup.py

@@ -190,7 +190,20 @@ greenlet_requires = [
     'greenlet >= 0.4.13; platform_python_implementation=="CPython"',
 ]
 
-install_requires = greenlet_requires + []
+# Note that we don't add cffi to install_requires, it's
+# optional. We tend to build and distribute wheels with the CFFI
+# modules built and they can be imported if CFFI is installed.
+# We need cffi 1.4.0 for new style callbacks;
+# we need cffi 1.11.3 (on CPython 3) to avoid test errors.
+
+# The exception is on Windows, where we want the libuv backend we distribute
+# to be the default, and that requires cffi
+cffi_requires = [
+    "cffi >= 1.11.5 ; sys_platform == 'win32'",
+]
+
+
+install_requires = greenlet_requires + cffi_requires
 
 # We use headers from greenlet, so it needs to be installed before we
 # can compile. If it isn't already installed before we start
@@ -204,7 +217,7 @@ install_requires = greenlet_requires + []
 # to install the headers at all, AFAICS, we don't need to bother with
 # the buggy setup_requires.)
 
-setup_requires = []
+setup_requires = cffi_requires + []
 
 if PYPY:
     # These use greenlet/greenlet.h, which doesn't exist on PyPy
@@ -249,18 +262,6 @@ for mod in _to_cythonize:
     EXT_MODULES.append(cythonize1(mod))
 del _to_cythonize
 
-try:
-    cffi = __import__('cffi')
-except ImportError:
-    pass
-else:
-    # Note that we don't add cffi to install_requires, it's
-    # optional. We tend to build and distribute wheels with the CFFI
-    # modules built and they can be imported if CFFI is installed.
-    # We need cffi 1.4.0 for new style callbacks;
-    # we need cffi 1.11.3 (on CPython 3) to avoid test errors
-    # install_requires.append('cffi >= 1.4.0')
-    pass
 
 if IGNORE_CFFI and not PYPY:
     # Allow distributors to turn off CFFI builds