Simple changes to address reviewer comments

Re-organize some imports, use run_in_tmpdir fixture,
use FutureWarning over DeprecationWarning

openmc/deplete/abc.py

@@ -80,7 +80,7 @@ class TransportOperator(metaclass=ABCMeta):
             else:
                 warn("Use of OPENMC_DEPLETE_CHAIN is deprecated in favor "
                      "of adding depletion_chain to OPENMC_CROSS_SECTIONS",
-                     DeprecationWarning)
+                     FutureWarning)
         self.chain = Chain.from_xml(chain_file)
 
     @abstractmethod

tests/unit_tests/test_deplete_operator.py

@@ -4,21 +4,20 @@ Modifies and resets environment variable OPENMC_CROSS_SECTIONS
 to a custom file with new depletion_chain node
 """
 
-from os import remove
 from os import environ
 from unittest import mock
 from pathlib import Path
-import pytest
 
+import pytest
 from openmc.deplete.abc import TransportOperator
 from openmc.deplete.chain import Chain
 
 BARE_XS_FILE = "bare_cross_sections.xml"
-CHAIN_PATH = Path().cwd() / "tests" / "chain_simple.xml"
+CHAIN_PATH = Path(__file__).parents[1] / "chain_simple.xml"
 
 
-@pytest.fixture(scope="module")
-def bare_xs():
+@pytest.fixture()
+def bare_xs(run_in_tmpdir):
     """Create a very basic cross_sections file, return simple Chain.
 
     """
@@ -33,7 +32,6 @@ def bare_xs():
         out.write(bare_xs_contents)
 
     yield
-    remove(BARE_XS_FILE)
 
 
 class BareDepleteOperator(TransportOperator):