Bump MSRV to 1.37

[taiki-e]

Bumps MSRV to 1.37 because underscore_const_names requires 1.37. This allows removing the build script.

It's been more than a year since 1.37 was released, and most of the users of this crate depend on futures_api that requires 1.36 and async_await that requires 1.39. so bumping MSRV to 1.37 is probably no problem.
Also, given that pin-project-lite's MSRV is 1.37 and the possibility of switching, most futures-related utilities require at least 1.37.

cc #264
Refs: #219

[taiki-e]

As far as I know, futures/futures-util/futures-test with no-default-features are the only futures-related utilities that have been tested to compatibility with 1.36 and depend on pin-project.
However, this should be fine as that is undocumented and there is a possibility of switching to pin-project-lite. (also probably many users have the default or async-await feature enabled.)

[taiki-e]

bors r+

See also #264 (comment)

[bors]

Build succeeded:

• ci