Move dbus-python from install_requires to extras_require
Move dbus-python from install_requires to extras_require

Installing it from PyPI can be difficult on some systems because it
requires a C compiler and the D-Bus headers. Installing it from distro
packages is much easier. Fixes #4.

Also, it turns out that installations from PyPI can be non-functional
sometimes (see jaraco/keyring#230, jaraco/keyring#231).

mitya57/secretstorage