Drop python3.7

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (174c21a) to head (0a06b6a). Report is 2 commits behind head on master.

:exclamation: Current head 0a06b6a differs from pull request most recent head 5d6bfc9

Please upload reports for the commit 5d6bfc9 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #61 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 26 26 Lines 1990 1977 -13 ========================================= - Hits 1990 1977 -13 ```

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[abersheeran]

Waiting https://github.com/mypyc/mypyc/issues/892