gh-136616: Improve assert syntax error messages

[sobolevn]

Local testing of positions and code highlight:

• Issue: assert does not have = -> == suggestion #136616

[sobolevn]

[lysnikolaou]

Is the second alternative here needed in reality. If someone's using the second argument here and have a comparison there, maybe they're confused and meant something completely different?

[sobolevn]

Hm, good question. They might. For example, assert some.method() == other.method(), some == other might be a realiastic case. But, I am open to other opinions :)

[sobolevn]

 trim trailing whitespace.................................................Failed
- hook id: trailing-whitespace
- exit code: 1
- files were modified by this hook

Fixing Grammar/python.gram

This is a bit annoying. I opened #136680 to fix this.