Add π to the math module

[JosephSBoyle]

Currently, the math module in CPython uses the identifier math.pi to represent the mathematical constant π. This breaks user expectations and violates the principle of least surprise.

To improve consistency and readability, I propose changing math.pi to math.π symbol throughout the module. The reasoning for this is threefold:

1. This change will make the code more intuitive for mathematicians and scientists (at least 99%+ of python users) who are more used to seeing the π symbol, thus reducing cognitive load and allowing them to build more complicated research code.

2. Python supports unicode characters such as π and we don't take advantage of this enough. We can use π, therefore we should.

3. Nobody currently knows how to type π, if we make this change users will have to open the math module and copy and paste the variable name - which is a great way to increase the visibility of important module internals that users need to read.

Some would be tempted to create a math.pi alias to maintain backwards compatibility, but I think that would only encourage people to continue using the non-symbolic variable (bad), so I propose simply deleting math.pi entirely (users will get the hint!).

This is a pretty small change so I don't think a news entry is necessary, in fact I think not having a news entry would make this change a sort of cool Easter Egg that users can discover and enjoy!

Kind regards and happy April 1st,
Joe