diffsptk version 0.6.0

What's Changed

• Support sinusoidal excitation by @takenori-y in #15
• Get alpha from sample rate by @takenori-y in #16
• Add root_pol by @takenori-y in #17
• Add vq by @takenori-y in #19
• Add msvq and imsvq by @takenori-y in #20
• Add unframe by @takenori-y in #21
• Add istft by @takenori-y in #22
• Add lbg by @takenori-y in #23
• Fix slow import by @takenori-y in #24
• Module sopr was stripped