Add support for alternative QR library

[bluetech]

Problem

django-otp currently optionally depends on the qrcode package for SVG QR image support.
My issue with qrcode is:

• Since version 7.4 it has a dependency on pypng (pypng maybe shouldn't be a hard dependency? lincolnloop/python-qrcode#312) which I personally like to avoid.

• Versions before 7.4 don't have a wheel, so I can't disable sdist building for security reasons if I use them instead.

Proposed solution

I would like to use a different QR library. I can see two alternatives for doing it:

1. Replace the qrcode dependency with another library
2. Add support for a second library as another optional dependency under a separate extra

For me both solutions would fix the issue. The second is less disruptive, but increases the testing matrix.

After looking into the available QR library options, segno looks to me like the best option. It is maintained, pure Python, can generate SVG and has zero dependencies.

If this proposal is accepted, I can send a PR for either option 1 or 2.

[psagers]

Adding segno support is fine with me. It's a close call, but probably best to minimize disruption. Perhaps we need a qrcode helper somewhere that will look for a compatible library. We could also add django-otp[segno] as a convenience.