Add mypy support

[cunla]

Fix #63

[github-actions]

Coverage report:

Name                                              Stmts   Miss Branch BrPart  Cover
-----------------------------------------------------------------------------------
fakeredis/__init__.py                                 6      2      0      0    67%
fakeredis/_basefakesocket.py                        218     12     94     11    92%
fakeredis/_commands.py                              259      7     86      4    96%
fakeredis/_fakesocket.py                             18      0      0      0   100%
fakeredis/_helpers.py                               188      7     67      3    96%
fakeredis/_msgs.py                                   51      0      0      0   100%
fakeredis/_server.py                                123      6     42      5    93%
fakeredis/_zset.py                                   64      0     16      0   100%
fakeredis/aioredis.py                               150      6     43      5    94%
fakeredis/commands_mixins/__init__.py                 0      0      0      0   100%
fakeredis/commands_mixins/bitmap_mixin.py            44      1      8      1    96%
fakeredis/commands_mixins/connection_mixin.py        20      0      4      0   100%
fakeredis/commands_mixins/generic_mixin.py          260     17    120     10    92%
fakeredis/commands_mixins/hash_mixin.py              82      1     16      1    98%
fakeredis/commands_mixins/list_mixin.py             176      5     76      5    96%
fakeredis/commands_mixins/pubsub_mixin.py            59      0     24      5    94%
fakeredis/commands_mixins/scripting_mixin.py        134      4     74      5    96%
fakeredis/commands_mixins/server_mixin.py            49      0     14      0   100%
fakeredis/commands_mixins/set_mixin.py              139     12     58      6    88%
fakeredis/commands_mixins/sortedset_mixin.py        320      4    137      3    98%
fakeredis/commands_mixins/string_mixin.py           164      3     64      3    97%
fakeredis/commands_mixins/transactions_mixin.py      73      2     20      1    97%
-----------------------------------------------------------------------------------
TOTAL                                              2597     89    963     68    95%