Fix tests breaking due to pytest mock

Release 1.12.0 of pytest-mock now raises an error on context manager
usage.
See https://github.com/pytest-dev/pytest-mock/blob/master/CHANGELOG.rst#1120-2019-11-19

test/test_filesystem.py

@@ -33,7 +33,7 @@ def test_read_file_missing_file():
 def test_read_file_cannot_read_ioerror(test_file, mocker):
     error = "foobar"
 
-    with mocker.patch("__builtin__.open", side_effect=IOError(error)):
+    mocker.patch("__builtin__.open", side_effect=IOError(error))
     with pytest.raises(MoulinetteError) as exception:
         read_file(str(test_file))
 
@@ -51,7 +51,7 @@ def test_read_json(test_json):
 def test_read_json_cannot_read(test_json, mocker):
     error = "foobar"
 
-    with mocker.patch("json.loads", side_effect=ValueError(error)):
+    mocker.patch("json.loads", side_effect=ValueError(error))
     with pytest.raises(MoulinetteError) as exception:
         read_json(str(test_json))
 
@@ -77,7 +77,7 @@ def test_write_to_new_file(tmp_path):
 def test_write_to_existing_file_bad_perms(test_file, mocker):
     error = "foobar"
 
-    with mocker.patch("__builtin__.open", side_effect=IOError(error)):
+    mocker.patch("__builtin__.open", side_effect=IOError(error))
     with pytest.raises(MoulinetteError) as exception:
         write_to_file(str(test_file), "yolo\nswag")
 
@@ -142,7 +142,7 @@ def text_write_list_to_json(tmp_path):
 def test_write_to_json_bad_perms(test_json, mocker):
     error = "foobar"
 
-    with mocker.patch("__builtin__.open", side_effect=IOError(error)):
+    mocker.patch("__builtin__.open", side_effect=IOError(error))
     with pytest.raises(MoulinetteError) as exception:
         write_to_json(str(test_json), {"a": 1})
 
@@ -165,7 +165,7 @@ def test_remove_file(test_file):
 def test_remove_file_bad_perms(test_file, mocker):
     error = "foobar"
 
-    with mocker.patch("os.remove", side_effect=OSError(error)):
+    mocker.patch("os.remove", side_effect=OSError(error))
     with pytest.raises(MoulinetteError) as exception:
         rm(str(test_file))