0.4.3 pins jsonschema <4.1.8, but it should be <4.18

[EliahKagan]

#235 correctly notes incompatibility with jsonschema 4.18 and recommends pinning <4.18, which appears to have been the intention of #236. But #236 pins <4.1.8 instead. This keeps it much farther back than necessary, which based on context appears unintended.

The effect is that installing python-jsonschema-objects 0.4.3 requires a downgrade to jsonschema 4.1.2 (there was no 4.1.8 version), which came out more than a year before the latest known-compatible version, 4.17.3.

I've opened PR #238 to change the pinning in setup.py from <4.1.8 to <4.18.

[cwacek]

In addition to accepting @EliahKagan's MR fixing this issue, I have deleted the 0.4.3 tag from Github and "yanked" it from PyPi, so hopefully people won't run into this issue just because they do a pip install unless they explicitly specify that version.