Initialise backend on first use instead of on import

[takluyver]

This seems so simple I kind of think I must be missing something, but tox passes on my (Linux) system.

Doing complicated stuff on import is generally best avoided, because we typically assume that imports are fast and reliable. This simply defers selecting and initialising a backend to the first time any of the public API functions are called. The only possible negative impact I can think of is if the environment changes between import and first use, it may lead to using a different backend. But it's hard to come up with realistic cases where that would be a problem, and if it is, the caller can use get_keyring() to initialise the backend early.

Closes #403.

[takluyver]

@jaraco can I interest you in this? The actual diff is pretty small.

[jaraco]

I like it. Yeah, let's do it.

[jaraco]

Releasing as 21.6.

[takluyver]

Thanks! 🤞 this doesn't break everything. 😉

[jyn514]

Wow, this really made an enormous difference ❤️ With the same commands as #403 (comment) this brought the import time down from 2 seconds to 200 milliseconds :) with a hot disk cache it's only 35 milliseconds.