Updated context to select default storage engine based on version.

Replicates kaizendorks#119

Author: infinityredux

pymongo_inmemory/context.py

@@ -5,7 +5,7 @@
 from os import path
 import platform
 
-from ._utils import mkdir_ifnot_exist
+from ._utils import mkdir_ifnot_exist, make_semver
 from .downloader._urls import best_url
 
 DEFAULT_CONF = {}
@@ -127,7 +127,7 @@ def __init__(
         )
         self.archive_folder = mkdir_ifnot_exist(self.download_folder, self.url_hash)
         self.extracted_folder = mkdir_ifnot_exist(self.extract_folder, self.url_hash)
-        self.storage_engine = conf("storage_engine", "ephemeralForTest")
+        self.storage_engine = self._build_storage_engine()
 
     def __str__(self):
         return (
@@ -165,3 +165,8 @@ def _build_download_url(self):
 
         self.downloaded_version = downloaded_version
         return dl_url
+
+    def _build_storage_engine(self):
+        major, minor, patch = make_semver(self.downloaded_version)
+        storage_engine_fallback = "wiredTiger" if major > 6 else "ephemeralForTest"
+        return conf("storage_engine", storage_engine_fallback)