Better understanding of operator precedence in colorizer

[tristanlatr]

Use parenthesis in colorized AST only if required. The only exceptions are in the presentation of the power operator and (of course) unsupported nodes, which uses astor to generate the code string. And astor gives surrounding parenthesis quite often.

Fixes #711

As a drive-by change, I've also fixed the colorization of dict unpack, which wasn't supported.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02 :warning:

Comparison is base (301d239) 92.60% compared to head (df0268a) 92.59%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #713 +/- ## ========================================== - Coverage 92.60% 92.59% -0.02% ========================================== Files 47 47 Lines 8129 8138 +9 Branches 1942 1945 +3 ========================================== + Hits 7528 7535 +7 - Misses 345 346 +1 - Partials 256 257 +1 ``` | [Impacted Files](https://app.codecov.io/gh/twisted/pydoctor/pull/713?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted) | Coverage Δ | | |---|---|---| | [pydoctor/epydoc/markup/\_pyval\_repr.py](https://app.codecov.io/gh/twisted/pydoctor/pull/713?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted#diff-cHlkb2N0b3IvZXB5ZG9jL21hcmt1cC9fcHl2YWxfcmVwci5weQ==) | `92.76% <100.00%> (-0.22%)` | :arrow_down: |

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[tristanlatr]

I’ll merge this PR next week if there is still no review.