circular imports

[Volker Braun]

There seem to be issues with circular imports. See http://trac.sagemath.org/15130 for details. Here is an example:

┌────────────────────────────────────────────────────────────────────┐

│ Sage Version 5.12.rc1, Release Date: 2013-10-04                    │

│ Type "notebook()" for the browser-based notebook interface.        │

│ Type "help()" for help.                                            │

└────────────────────────────────────────────────────────────────────┘

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┃ Warning: this is a prerelease version, and it may be unstable.     ┃

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sage: from sage.matroids.basis_exchange_matroid import BasisExchangeMatroid

---------------------------------------------------------------------------

AttributeError                            Traceback (most recent call last)

<ipython-input-1-2e7708db611e> in <module>()

----> 1 from sage.matroids.basis_exchange_matroid import BasisExchangeMatroid

/home/vbraun/Code/sage.git/basis_exchange_matroid.pyx in init sage.matroids.basis_exchange_matroid (sage/matroids/basis_exchange_matroid.c:20420)()

/home/vbraun/Code/sage.git/basis_exchange_matroid.pxd in init sage.matroids.matroid (sage/matroids/matroid.c:33184)()

AttributeError: 'module' object has no attribute 'BasisExchangeMatroid'