limit greenlet dependency to pypi-listed platforms

The setup requirements have been modified such ``greenlet`` is a default
requirement only for those platforms that are well known for ``greenlet``
to be installable and for which there is already a pre-built binary on
pypi; the current list is ``x86_64 aarch64 ppc64le amd64 win32``. For other
platforms, greenlet will not install by default, which should enable
installation and test suite running of SQLAlchemy 1.4 on platforms that
don't support ``greenlet``, excluding any asyncio features. In order to
install with the ``greenlet`` dependency included on a machine architecture
outside of the above list, the ``[asyncio]`` extra may be included by
running ``pip install sqlalchemy[asyncio]`` which will then attempt to
install ``greenlet``.

Additionally, the test suite has been repaired so that tests can complete
fully when greenlet is not installed, with appropriate skips for
asyncio-related tests.

Fixes: #6136
Change-Id: I8f3a1c00a4a8b6a273484af1da1f7aaadf588ae7

1 files changed, 12 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/requirements.py b/lib/sqlalchemy/testing/requirements.py
index 702a40fe2..4fa701b10 100644
--- a/lib/sqlalchemy/testing/requirements.py
+++ b/lib/sqlalchemy/testing/requirements.py
@@ -1320,6 +1320,18 @@ class SuiteRequirements(Requirements):
         return exclusions.closed()
 
     @property
+    def greenlet(self):
+        def go(config):
+            try:
+                import greenlet  # noqa F401
+            except ImportError:
+                return False
+            else:
+                return True
+
+        return exclusions.only_if(go)
+
+    @property
     def computed_columns(self):
         "Supports computed columns"
         return exclusions.closed()