Fix unit tests for python 3.8

src/charm.py

@@ -5,6 +5,7 @@
 import logging
 import os
 import subprocess
+from os.path import islink
 
 from jinja2 import Template
 from ops.charm import CharmBase
@@ -85,7 +86,7 @@ def _render_config(self, config):
         with open("/etc/nginx/sites-available/{}".format(site_conf_name), "wb") as f:
             b = t.render(config=config).encode("UTF-8")
             f.write(b)
-        if not os.path.islink("/etc/nginx/sites-enabled/{}".format(site_conf_name)):
+        if not islink("/etc/nginx/sites-enabled/{}".format(site_conf_name)):
             os.symlink(
                 "/etc/nginx/sites-available/{}".format(site_conf_name),
                 "/etc/nginx/sites-enabled/{}".format(site_conf_name),

tests/unit/test_charm.py

@@ -116,17 +116,16 @@ def test_reload_config(self, mock_subproc):
         harness.charm._reload_config()
         assert mock_subproc.call_args == call(["service", "nginx", "restart"])
 
-    @patch("os.path.islink")
+    @patch("charm.islink")
     @patch("os.symlink")
     @patch("subprocess.check_call")
     def test_render_config(self, mock_subproc, os_symlink, os_path_islink):
         harness = Harness(NginxCharm)
         self.addCleanup(harness.cleanup)
         harness.begin()
         config = {}
-        mock_open_call = mock_open(read_data="")
         os_path_islink.return_value = False
-        with patch("builtins.open", mock_open_call):
+        with patch("builtins.open", mock_open()) as mock_open_call:
             harness.charm._render_config(config)
         assert mock_open_call.call_args_list[0] == call("templates/nginx.conf.j2")
         assert mock_open_call.call_args_list[1] == call("/etc/nginx/nginx.conf", "wb")