Fix bug #1565514, SystemError not raised on too many nested blocks.
It seems like this should be a different error than SystemError, but
I don't have any great ideas and SystemError was raised in 2.4 and earlier.
Expand set of errors caught in set_context(). Some new errors, some
old error messages changed for consistency.
Fixed error checking in generator expression code. The first set of
tests were impossible condition given the grammar. In general, the
ast code uses REQ() for those sanity checks.
Fix some error handling for augmented assignments. As comments in the
code explain, set_context() ought to work here, but I got unexpected
crashes when I tried it. Should come back to this.
Add note to Grammar that yield expression is a special case.
Add doctest cases for SyntaxErrors raised by ast.c.
imports e.g. test_support must do so using an absolute package name
such as "import test.test_support" or "from test import test_support".
This also updates the README in Lib/test, and gets rid of the
duplicate data dirctory in Lib/test/data (replaced by
Lib/email/test/data).
Now Tim and Jack can have at it. :)