Make sympy.abc more understandable

[toolforger]

Generating symbols through the compiler is not understood by analysis tools,
so they will generate an error for every use of something imported from
sympy.abc.

[toolforger]

test_import reports 0.220456 +- 0.002123 before this change, 0.220669 +- 0.002250 after.
Error/warning count in Eclipse PyDev is 7752/3261 before, 7242/3261 after, a difference of -510/+1.

[smichr]

FYI: don't forget about the rage capability of symbols, too:

A, B, C, D, E, F, G, H, I, J, K, L, M = symbols('A:M')
N, O, P, Q, R, S, T, U, V, W, X, Y, Z = symbols('N:Z')
a, b, c, d, e, f, g, h, i, j, k, l, m = symbols('a:m')
n, o, p, q, r, s, t, u, v, w, x, y, z = symbols('n:z')

[toolforger]

Ah, I didn't think about that.
I made it like a, b, c, d, e = symbols('a, b, c, d, e') because I wanted to show off how the right-hand side can be a copy of the left-hand side.

Comparing the approaches, it seems to me that 'a:m' is more elegant, 'a, b, c, d, e' is the better precedent. Barring aspects I overlooked, I prefer the precedent.

I added a comment in the code to explain that, so people will know why it's the way it is.
(That's just in case you agree, so you can review the wording right away.)

[jcrist]

This looks fine to me. @smichr: thoughts?

[smichr]

+1