Fix all warnings (#449)

* fix typo in pytest.mark.xskip

Change xskip by skipif marker as xskip is an unknown pytest marker.

* fix FileModeWarning

This fix the following warning:
FileModeWarning: Requests has determined the content-length for this request using the binary size of the file: however, the file has been opened in text mode (i.e. without the 'b' flag in the mode). This may lead to an incorrect content-length. In Requests 3.0, support will be removed for files in text mode.

* fix waring "calling yaml.load() without Loader=... is deprecated"
This fix the following warning:
YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.

* fix collections.abc deprecation warning in python 3.7.

* update Flask dependency in order to get rid of the Request.is_xhr warning

This fix the following warning:
DeprecationWarning: 'Request.is_xhr' is deprecated as of version 0.13 and will be removed in version 1.0. The 'X-Requested-With' header is not standard and is unreliable. You may be able to use 'accept_mimetypes' instead.

tests/unit/test_migration.py

@@ -5,6 +5,12 @@ import yaml
 
 import vcr.migration
 
+# Use the libYAML versions if possible
+try:
+    from yaml import CLoader as Loader
+except ImportError:
+    from yaml import Loader
+
 
 def test_try_migrate_with_json(tmpdir):
     cassette = tmpdir.join('cassette.json').strpath
@@ -22,9 +28,9 @@ def test_try_migrate_with_yaml(tmpdir):
     shutil.copy('tests/fixtures/migration/old_cassette.yaml', cassette)
     assert vcr.migration.try_migrate(cassette)
     with open('tests/fixtures/migration/new_cassette.yaml', 'r') as f:
-        expected_yaml = yaml.load(f)
+        expected_yaml = yaml.load(f, Loader=Loader)
     with open(cassette, 'r') as f:
-        actual_yaml = yaml.load(f)
+        actual_yaml = yaml.load(f, Loader=Loader)
     assert actual_yaml == expected_yaml