Introduce roundtrip testing

[kurtmckee]

While working on #206, I kept encountering bugs and missing features in the existing implementation's JsonPath string serialization and equality methods that hampered comparison and roundtrip testing. I'm therefore submitting this PR to fix bugs in the existing implementation.

This PR introduces the following changes:

• Add hypothesis as a testing tool
• Add roundtrip testing: given an arbitrary string that can be parsed, it should be possible to parse the string, serialize the JsonPath instance back to a string, and re-parse that string back to the same JsonPath instance.

I had hypothesis generate ~10,000,000 examples cumulatively, which identified a number of issues in the current implementation:

---

Added

• Support equality checking of Operation instances
• Support string serialization of Union and Intersect instances

Fixed

• Fix string serialization throughout the library to enforce roundtrip parsing consistency.
  • Fields are more conservatively enclosed in quotion marks.
    This fixes serialization and re-parsing of "00", '%', '0@' and "&'".
  • Operation instances can now be serialized.
    This fixes serialization of 0-@ and A -A.
  • SortedThis instances can now be serialized and re-parsed.
    This fixes serialization of 0[/0].
  • Child precedence is now preserved using parentheses during serialization.
    This ensures that serialized strings like a..b[c] serialize and re-parse identically.
• Fix parsing and string serialization of numeric-only identifiers.
  This fixes parsing of 10, which was parsed as two separate fields.
• Fix equality checks for SortedThis instances.

[michaelmior]

LGTM, just one minor suggestion.

[kurtmckee]

@michaelmior Is there anything else that needs to be addressed or considered before this can be merged?

[michaelmior]

@kurtmckee Nope! Go for it :) (I guess a minor merge conflict changelog needs to be resolved, but obviously not a big deal.)

[kurtmckee]

I don't have merge permissions on the project. 👍

I'll fix the merge conflict straight away, though!

[kurtmckee]

Rebased on master and force-pushed to resolve merge conflicts.

I also fixed a typo in the CHANGELOG (the word "pickly" -> "pickle") and removed trailing whitespace.

[michaelmior]

Merged! Thanks :)

[lantrix]

Any chance of a release to now remove your use of the deprecated ply@3.11 package?

https://security.snyk.io/vuln/SNYK-PYTHON-PLY-15046763
https://github.com/bohmiiidd/Undocumented-RCE-in-PLY