Move version from const.py to __init__.py for Flit static introspection

[CAM-Gerlach]

As discussed in #164 , moving __version__ from const.py to be directly in __init__,py allows Flit to statically introspect it, preventing requests from being required as an unspecified, implict build-time dependency, causing build/install failures in certain scenarios. I've tested and confirmed locally that this fix allows successfully building and installing prawcore without having requests in the local environment.

Additionally, I've updated the one internal module using this value from const.py to use that from __init__.py. I did have to switch from a relative from import to an absolute import to avoid an import cycle error; alternatively, I could also move the import to be inside the relevant function, though I figured the former would be more acceptable/less problematic for you—happy to change that as desired.

Finally, I updated the version bump script in tools/set_version.py to bump the version in its new location.

[LilSpazJoekp]

Yeah let's do a local import.

[LilSpazJoekp]

Let's do a local relative import.

[CAM-Gerlach]

Sure, done as requested 👍

[LilSpazJoekp]

Thanks for this!

Would you be interested in doing the same for PRAW, Async PRAW, and Asyncprawcore?

[CAM-Gerlach]

Hey, sorry, was looking into this, but ran into a couple issues and then it fell through the cracks.

I opened praw-dev/praw#2004 to cover this in PRAW and discuss the implementation there; TL;DR we could do a similar approach, but it does require changing one other const.py module-level constant in a backward-incompatible way, so we may want to re-consider Approach 2 (static metadata) here, but that can be discussed over there.

As for asyncpraw and asyncprawcore, at the time I first looked into this request they were both still using Setuptools, but I see they are now using Flit, so they do indeed need this too, presumably. Asyncpraw runs into the same issue as PRAW with USER_AGENT_FORMAT and can be fixed the same way, with the same impact (albeit theoretical in this case); however, I note one extant usage of asyncpraw.const.__version__ in the wild, in someone's Reddit bot with a handful of stars. Approach 2 (static metadata) would avoid this as well, as well as simplify the implementation there since it would essentially just require the same changes to set_version.py and pyproject.toml and that's it.