Fixed parsing precedence and associativity

[vallentin]

Fixes #381

This pull request fixes the parser, such that it produces a correct AST in relation to precedence and associativity.

As mentioned in the issue, given that Askama emits the expressions unmodified, then in that regard there isn't any noticeable change. However, with a correct AST it prevents any potential future problems.

In short a + b == c was previously parsed as (a + (b == c)) and now is correctly parsed as ((a + b) == c).

I also added a few test cases to check that the parser upholds precedence and associativity. (If you want the test cases split into multiple functions, then mention it and I'll split them.)

This does not introduce any breaking changes.

[djc]

Awesome, thanks for fixing this! I'll merge as soon as this passes CI.

[vallentin]

Fixing the issue.