setup.py: fix pyxdg requirement

xdg is declaring itself as pyxdg.

Detected when installing through flatpak. It tries installing 'xdg'
through pip while we already manually installed it:

Processing dependencies for tuhi==0.1
Searching for xdg
Reading https://pypi.python.org/simple/xdg/
Download error on https://pypi.python.org/simple/xdg/: [Errno -3] Temporary failure in name resolution -- Some packages may not be found!
Couldn't find index page for 'xdg' (maybe misspelled?)
Scanning index of all packages (this may take a while)
Reading https://pypi.python.org/simple/
Download error on https://pypi.python.org/simple/: [Errno -3] Temporary failure in name resolution -- Some packages may not be found!
No local packages or working download links found for xdg
error: Could not find suitable distribution for Requirement.parse('xdg')
Error: module tuhi: Child process exited with code 1

setup.py

@@ -23,7 +23,7 @@ setup(name='tuhi',
       python_requires='>=3.6',
       install_requires=[
           'svgwrite',
-          'xdg',
+          'pyxdg',
           'pygobject',
       ]
       )