Use Sequence instead of List to annotate the Class._mro attribute

[tristanlatr]

Fix #638

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (39a34ca) 92.51% compared to head (2c1d029) 92.51%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #706 +/- ## ======================================= Coverage 92.51% 92.51% ======================================= Files 47 47 Lines 8126 8126 Branches 1942 1942 ======================================= Hits 7518 7518 Misses 354 354 Partials 254 254 ``` | [Impacted Files](https://app.codecov.io/gh/twisted/pydoctor/pull/706?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted) | Coverage Δ | | |---|---|---| | [pydoctor/model.py](https://app.codecov.io/gh/twisted/pydoctor/pull/706?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted#diff-cHlkb2N0b3IvbW9kZWwucHk=) | `94.63% <100.00%> (ø)` | |

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[tristanlatr]

This should be trivial to review