Timeseries branch won't build with Python 3.
I got the following error when trying to build the timeseries branch with python-3.2:

cythoning pandas/src/tseries.pyx to pandas/src/tseries.c
## Error compiling Cython file:

...
    to use start ('S') or end ('E') of interval.
    """
    cdef:
        long retval

```
if not isinstance(relation, basestring) or len(relation) != 1:
                                     ^
```

---

pandas/src/datetime.pyx:961:42: undeclared name not builtin: basestring
building 'pandas._tseries' extension
gcc -pthread -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -I/usr/local/lib/python3.2/site-packages/numpy/core/include -I/usr/local/include/python3.2m -c pandas/src/tseries.c -o build/temp.linux-x86_64-3.2/pandas/src/tseries.o
pandas/src/tseries.c:1:2: error: #error Do not use this file, it is the result of a failed Cython compilation.

Replacing all uses of "basestring" in tseries.pyx with "str" allows it to build. (I'm new to python, so I'm not sure whether this solution is appropriate.)
