fix test dependencies

Author: shaypal5

tests/requirements.txt

@@ -7,3 +7,5 @@ birch
 # to be able to run `python setup.py checkdocs`
 collective.checkdocs
 pygments
+# the memory core tests dataframe caching
+import pandas

tests/test_mongo_core.py

@@ -1,43 +1,54 @@
 """Testing the MongoDB core of cachier."""
 
+# standard library imports
 import datetime
 import hashlib
 import platform
 import queue
 import sys
 import threading
-import warnings
 from random import random
 from time import sleep
 from urllib.parse import quote_plus
 
+# third-party imports
+import pytest
+from birch import Birch  # type: ignore[import-not-found]
 try:
     import pandas as pd
 except (ImportError, ModuleNotFoundError):
     pd = None
-    warnings.warn("pandas is not installed; tests requiring pandas will fail!")
+    print("pandas is not installed; tests requiring pandas will fail!")
 
 try:
     import pymongo
     from pymongo.errors import OperationFailure
     from pymongo.mongo_client import MongoClient
-    from pymongo_inmemory import MongoClient as InMemoryMongoClient
 except (ImportError, ModuleNotFoundError):
+    print("pymongo is not installed; tests requiring pymongo will fail!")
     pymongo = None
-    MongoClient = None
-    InMemoryMongoClient = None
     OperationFailure = None
-    warnings.warn(
-        "pymongo is not installed; tests requiring pymongo will fail!"
-    )
-import pytest
-from birch import Birch  # type: ignore[import-not-found]
+    # define a mock MongoClient class that will raise an exception
+    # on init, warning that pymongo is not installed
+    class MongoClient:
+        def __init__(self, *args, **kwargs):
+            raise ImportError("pymongo is not installed!")
+try:
+    from pymongo_inmemory import MongoClient as InMemoryMongoClient
+except (ImportError, ModuleNotFoundError):
+    class InMemoryMongoClient:
+        def __init__(self, *args, **kwargs):
+            raise ImportError("pymongo_inmemory is not installed!")
+    print("pymongo_inmemory is not installed; "
+          "in-memory MongoDB tests will fail!")
 
+# local imports
 from cachier import cachier
 from cachier.config import CacheEntry
 from cachier.cores.base import RecalculationNeeded
 from cachier.cores.mongo import _MongoCore
 
+
 # === Enables testing vs a real MongoDB instance ===