CI: use pytest append import mode so the installed wheel wins

In cibuildwheel the tests are run against the installed wheel, but pytest's
default 'prepend' import mode inserts the project root at sys.path[0]. That
shadows the installed compiled wheel with the in-tree pure-Python cassandra/
source, so 'import cassandra.row_parser' / 'cassandra.io.libevreactor' /
'cassandra.cmurmur3' all fail and every libev/cmurmur3/Cython test skips.
Under CASS_DRIVER_NO_SKIP those skips became failures. Switch to
--import-mode=append so site-packages (the wheel) takes precedence while the
'tests' package is still importable from the appended project root.

Author: sylwiaszunejko

pyproject.toml

@@ -121,7 +121,12 @@ log_format = "%(asctime)s.%(msecs)03d %(levelname)s [%(module)s:%(lineno)s]: %(m
 log_level = "DEBUG"
 log_date_format = "%Y-%m-%d %H:%M:%S"
 xfail_strict = true
-addopts = "-rf"
+# Use 'append' import mode so the installed package (e.g. the compiled wheel
+# under cibuildwheel) takes precedence on sys.path over the in-tree ``cassandra``
+# source directory. With the default 'prepend' mode pytest inserts the project
+# root at sys.path[0], which shadows the installed wheel with the uncompiled
+# source tree and makes every C-extension test (libev, cmurmur3, Cython) skip.
+addopts = "-rf --import-mode=append"
 markers = [
     "last: mark test to run last within its module group",
 ]