Fix tspan not respecting text-anchor

[michaellisitsa]

text-anchor attribute on tspan is not respected. It previously worked in v60.1 however the logic for text-anchor was moved into the svg/__init__.py file and caused this edge case not be handled, and there were no specific test cases.

See example below where text-anchor="middle" doesn't do anything

Tests pass

Replit reproducing bug: https://replit.com/@mlisitsa/Python-Weasyprint
Example:

<svg
  width="600"
  height="217"
  role="img"
  viewBox="0 0 600 217"
  style="pointer-events: all; width: 100%; height: 100%"
>
  <text x="10" y="30" class="small">
    <tspan x="0" dx="0" dy="0" text-anchor="middle" >Middle</tspan>
    <tspan x="0" dx="0" dy="15">Start</tspan>
  </text>
</svg>

[grewn0uille]

Thanks!