use pkg_resources

This change uses if no path for e capabilities-file is supplied a
temporary file created by pkg_resources, which should be more robust
than directly accessing the file. (This failed sometimes, for example
in zipped distributions or uncommon structures)

doc/requirements.txt

@@ -3,6 +3,7 @@ Pillow>=2.0
 qrcode>=4.0
 pyserial
 sphinx-rtd-theme
+setuptools
 setuptools-scm
 docutils>=0.12
 python-barcode>=0.11.0,<1