Drop Python 3.5 support

[dmtucker]

$ pypinfo pytest-mypy pyversion
Served from cache: False
Data processed: 941.90 MiB
Data billed: 942.00 MiB
Estimated cost: $0.01

| python_version | download_count |
| -------------- | -------------- |
| 3.9            |      1,665,299 |
| 3.7            |         50,541 |
| 3.8            |         39,188 |
| 3.10           |         15,260 |
| 3.6            |          7,419 |
| 3.5            |            245 |
| 3.11           |            228 |
| 2.7            |              2 |
| 3.4            |              1 |
| Total          |      1,778,183 |