Detect single letter variable names enumerated in PEP-0008

[mtmiller]

Fix for #341. Could be improved further to include named parameters, maybe other cases I can't think of where names are declared.

[IanLee1521]

Thanks @mtmiller ! Could you update the doc tests with the rest of the cases that are not allowed?

Also, the 'as' test cases are missing.

[mtmiller]

Further doctests for all 3 forbidden variable names added, as well as some tests on except/as and with/as.

While hacking on this I realized I missed some other obvious cases such as for loop iterators and initialization of a list of variables such as I, J, K = ....

[mtmiller]

Added checks for names after class, def, global, and nonlocal, with new error IDs for function and class definitions versus local variables.

Now reports errors for almost everything in the original example in #341.

Still missing checks for for loop iterators, function parameters, and the comma-separated lists of variables that can appear in assignments, for loops, arg lists, and global and nonlocal statements.

[sigmavirus24]

This will have to be included in 2.1.0 or some other 2.x.0 release.

[IanLee1521]

Next version will be 2.1.0, going to go ahead and merge this. Thanks @mtmiller !