vips: Add optional dependencies; update to python3
vips: Add optional dependencies; update to python3

The new optional dependencies increase closure size from 388.92 MiB to
414.0 MiB, a negligible change. I didn't think there was any reason to
add `withLibjpeg ? true` flags (or `enableLibjpeg`).

The optional dependencies are sorted in the order of
https://github.com/libvips/libvips/blob/master/README.md
and I have used `nix log` on the build to check they are all picked up
in configure.

The python3 support was in for ages
(https://github.com/libvips/libvips/issues/202), the remaining python3
support in tools/vipsprofile landed in
https://github.com/libvips/libvips/pull/1262 so I upgraded it to
python3. I wasn't sure if I should use a specific version, or just leave
it as python3.

I have tested vipsprofile with
```
./vips-pycairo-bin/bin/vips gaussblur ~/2021-04-30-092143_3286x1080_scrot.png x.tif 5 --vips-profile
./vips-pycairo-bin/bin/vipsprofile
```
where ~/2021-04-30-092143_3286x1080_scrot.png is just a random image
(screenshot) I had lying around. It seems to have generated an SVG file,
I don't know how to interpret it.

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