0.7.3

This is the release pull request. **Do not merge this. I will merge it after the release is done.**

This is a continuation of https://github.com/sympy/sympy/pull/2253, which had to be merged prematurely. Here are the notes from that pull request:

This is the branch for the 0.7.3 release. Please make any any pull requests for changes that should go into the release against this branch. Only major changes should go in the release. All other changes should go in master. 

The primary work on this branch will be working on automating our release process with fabric and vagrant. See the release directory. If anyone wants to help out with this, just let me know. 

Here is a copy of the checklist from https://github.com/sympy/sympy/pull/2233:

See https://github.com/sympy/sympy/wiki/New-Release for a full list of things. Everything on that page falls into one of three categories

- Things that are test related. These should be put into Travis. This doesn't include things that are specific to releasing, like testing the tarball itself. Most of this work is already done, but a notable exception is that Travis doesn't test with any external dependencies other than gmpy.
- Things that need to be automated. These are listed above.
- Things that we should not be doing. This includes creating any package other than the source tarball, and updating websites that we don't own. 

Here is a checklist of things that do need to be automated. *Note* the checklist is not for things that have been done. It's for things that need to be *automated*. Some things have already been done for this release, but they need to be automated so that we can do it easily in the future. 

- [x] Make a branch, 0.7.3. 
- [x] Change "0.7.2-git" to "0.7.3.rc1" in `sympy/__init__.py`
- [x] Make sure the tests pass on this branch. I wouldn't focus on running the tests with vagrant (except for testing the release itself). It should poll Travis to see if the tests are passing. 
- [x] Build the source tarball
- [x] Make sure that everything is in the tarball
- [x] Make sure that nothing extra is in the tarball
- [x] All of the above for Python 3 too!
- [x] Make sure that AUTHORS and .mailmap are up to date (see the comment at the top of .mailmap). This can be skipped for this release because I have already done this manually.
- [ ] Build the release notes, which includes:
  - [ ] Gather all changes from pull requests. Obviously, this can't be completely automated, but it would be great if it could help as much as possible.
  - [x] Gather the list of AUTHORS who contributed to the release, and who has contributed the first time since this release.
  - [ ] Gather all deprecation related changes.
- [ ] Update all websites *that we own*. There are still a lot of websites here, so we should think how to remove some of the duplication. For example, I don't think we should bother updating stuff on Google Code any more. For updating the docs, some of the scripting should be on the docs repo (see e.g. https://github.com/sympy/sympy_doc/pull/3, from the SciPy sprints)
- [x] Upload to PyPI. We should upload only there. We can play with binstar too if you want, but this should be lower priority than everything else I have listed here.
- [ ] Building the docs. This means building the docs tarball, the pdf docs, uploading them to the right place (PyPI?), and updating docs.sympy.org. Last time, I also built the docs in Python 3, but I'm not sure if it was worth the effort (the main difference was the source pages, and also the doctests were run through 2to3). (We have building, but not uploading).
- [ ] Compose an email to the list and a blog post (I really do mean automate *everything*).

- [ ] **Before merging, revert 3d56d4b8c1d8346bc92bc23487cbb82214c6c476!**