Do not touch botocore unless it is installed (#803)

* Proper S3 exception on MISSING_DEPS

* Fix check for MISSING_DEPS

* Add test

.github/workflows/python-package.yml

@@ -51,6 +51,12 @@ jobs:
       - name: Upgrade PyOpenSSL
         run: python -m pip install pyOpenSSL --upgrade
 
+      - name: Install smart_open without dependencies
+        run: pip install -e .
+
+      - name: Check that smart_open imports without dependencies
+        run: python -c 'import smart_open'
+
       - name: Install smart_open and its dependencies
         run: pip install -e .[test]
 

smart_open/s3.py

@@ -118,7 +118,8 @@ def _do(self, fn: Callable):
 # The retry mechanism for this submodule.  Client code may modify it, e.g. by
 # updating RETRY.sleep_seconds and friends.
 #
-RETRY = Retry()
+if 'MISSING_DEPS' not in locals():
+    RETRY = Retry()
 
 
 class _ClientWrapper: