Update dependency pyxdg to v0.26 (#2823)

This PR contains the following updates:

| Package | Update | Change | References |
|---|---|---|---|
| pyxdg | minor | `==0.25` -> `==0.26` | [homepage](http://freedesktop.org/wiki/Software/pyxdg) |

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frozen_requirements.txt

@@ -96,7 +96,7 @@ PyNaCl==1.3.0
 pytest==3.8.2
 pytest-timeout==1.3.2
 python-dateutil==2.7.3
-pyxdg==0.25
+pyxdg==0.26
 PyYAML==3.13
 requests==2.20.0
 requests-oauthlib==1.0.0