fix weird test isolation errors with os.environ and boto3

test_config manipulates os.environ. os.environ is an `environ` object,
which acts like a dict but isn't in some subtle unknowable ways. The
`reload_config` fixture would create a dict copy of the env, and then
just call `os.environ = old_env` afterwards.

Boto3 would then complain that it couldn't load credentials (despite us
using the mock_s3 fixture and also not having creds in the environment
in the first place). Not entirely sure why this happens, but it does.
For some reason, it being a `dict` instead of an `environ` object causes
the mocking of boto3 to fail.

The solution is to not overwrite os.environ entirely, rather, use the
standard dictionary setitem syntax to update the values to their
previous values. Use `clear` to empty the environment too.

tests/app/test_config.py

@@ -21,7 +21,9 @@ def reload_config():
 
     yield
 
-    os.environ = old_env
+    for k, v in old_env.items():
+        os.environ[k] = v
+
     importlib.reload(config)
 
 

tests/conftest.py

@@ -136,9 +136,10 @@ def os_environ():
             assert type(value) == str
             super().__setitem__(key, value)
 
-    os.environ = EnvironDict()
+    os.environ.clear()
     yield
-    os.environ = old_env
+    for k, v in old_env.items():
+        os.environ[k] = v
 
 
 def pytest_generate_tests(metafunc):