additional typing improvements

[mgor]

when doing some "integration" work with RequestStats, i noticed that there was a lot of stuff that was missing types and had inferred Any.

so this PR started out with the goal to add typing to locust.stats which took me down a rabbit hole of also adding types in locust.env, locust.web, locust.runners and locust.user.

i had to get creative in some places, which added a bit more code than just the types, mostly due to how many variables has an initial value of None.

also, maybe a bit unnecessary... but StatsEntry had serialize and unserialize, while StatsError for the same thing had to_dict and from_dict, so changed it so they have the same names.

[cyberw]

Awesome. Just let me test it against my own locustfiles first.

[cyberw]

thx