1.x release plan

[paulmelnikow]

As I mentioned in #118 it would be good to get this library out of 0.x territory. We also have a few breaking changes to make.

The next release will be 1.0.0b0, and I'll keep releasing betas until we have a known working release with at least the following issues fixed:

• Require Python 3 (Require Python 3 #124)
• Don't allow missing snapshots without --snapshot-update (Way to fail in CI when running tests and the test writes a new snapshot #73)

[stinovlas]

Hi! I've noticed that there hasn't been any release since 2020, but the project still seems to be maintained. I bumped into a problem of snapshottest using deprecated imp library and I also found out that this has already been resolved in the code, just not released. Would you please consider publishing another sub-1 version (such as 0.7.0) that would allow using snapshottest with Python 3.12?