remove last user of SafeConfigParser

Fixes #5059

src/pip/_internal/vcs/mercurial.py

@@ -45,7 +45,7 @@ def fetch_new(self, dest, url, rev_options):
 
     def switch(self, dest, url, rev_options):
         repo_config = os.path.join(dest, self.dirname, 'hgrc')
-        config = configparser.SafeConfigParser()
+        config = configparser.RawConfigParser()
         try:
             config.read(repo_config)
             config.set('paths', 'default', url)

tests/functional/test_vcs_mercurial.py

@@ -0,0 +1,28 @@
+"""
+Contains functional tests of the Mercurial class.
+"""
+
+import os
+
+from pip._vendor.six.moves import configparser
+
+from pip._internal.vcs.mercurial import Mercurial
+from tests.lib import need_mercurial
+
+
+@need_mercurial
+def test_mercurial_switch_updates_config_file_when_found(tmpdir):
+    hg = Mercurial()
+    options = hg.make_rev_options()
+    hg_dir = os.path.join(tmpdir, '.hg')
+    os.mkdir(hg_dir)
+    config = configparser.RawConfigParser()
+    config.add_section('paths')
+    config.set('paths', 'default', 'old_url')
+    hgrc_path = os.path.join(hg_dir, 'hgrc')
+    with open(hgrc_path, 'w') as f:
+        config.write(f)
+    hg.switch(tmpdir, 'new_url', options)
+    config.read(hgrc_path)
+    default_path = config.get('paths', 'default')
+    assert default_path == 'new_url'