add arctan2 and curly brace validation

[ChristopherChudzicki]

Edit: This PR addresses #133 and #138

[jolyonb]

Want to update version numbers everywhere to 1.2.1? I think we should also include #138 in a version update.

Otherwise, this looks fine to me. Might suggest adding a couple of simple tests in the functions file where we test the inverse trig functions - arctan2(x, y) == arctan(y/x) type things.

[ChristopherChudzicki]

Sure, I can implement #138 in this PR and add it to changelog. Either later tonight or tomorrow.

I just updated version in README.

I moved the arctan2 tests into their own file because of https://bugs.python.org/issue6835

[jolyonb]

Looks good. Also remember to update __version__ in mitxgraders/version.py

[jolyonb]

All looks good 👍