Add --split option to split the tests and doctests

The usage is

./bin/test --split 1/2

which will run the first half of the tests. Any a/b is supported.

Currently, this just splits the test files, because that was the easiest to
do, but we maybe should implement splitting the tests themselves.

Note that for the doctests, the regular doctests and the Sphinx doctests are
split independently. Furthermore, the Sphinx doctests split against every
Sphinx file, before it checks if it actually has doctests, so for some splits,
no Sphinx doctests will be run at all.
Implement test splitting in Travis

Now the regular tests are split into two test runs.
Print the split in the test header if there is one

This will help in debugging why this doesn't seem to work in Travis.
Sort paths in the tests

This should make --split act as you would expect it to, regardless of the
underlying filesystem.
Travis: Also split the gmpy tests
Sort the right thing when running the tests

This reverts commit 6495de3c4584754644a9b7f565ec2d7b893fbbf9, because it was
sorting the wrong list.
Update list of installed modules
Merge pull request #2458 from asmeurer/split

Add --split option to split the tests and doctests