Support psycopg3

[nitrag]

It appears psycopg3 returns bytes by default.

[submarcos]

I, thank you for you contrib

I approve the PR to run tests, if you want to improve test matrix to keep new lines covered

[codecov[bot]]

Codecov Report

Patch coverage: 40.00% and project coverage change: -1.07 :warning:

Comparison is base (78d3c40) 100.00% compared to head (c56c77d) 98.93%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #41 +/- ## =========================================== - Coverage 100.00% 98.93% -1.07% =========================================== Files 12 12 Lines 279 283 +4 =========================================== + Hits 279 280 +1 - Misses 0 3 +3 ``` | [Impacted Files](https://app.codecov.io/gh/submarcos/django-vectortiles/pull/41?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jean-Etienne+Castagnede) | Coverage Δ | | |---|---|---| | [vectortiles/postgis/mixins.py](https://app.codecov.io/gh/submarcos/django-vectortiles/pull/41?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jean-Etienne+Castagnede#diff-dmVjdG9ydGlsZXMvcG9zdGdpcy9taXhpbnMucHk=) | `88.00% <40.00%> (-12.00%)` | :arrow_down: |

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[nitrag]

Honestly, I'm not sure where to right write the coverage test.

[submarcos]

replaced by #42