Unable to import trac.db_default due to circular imports on Trac 1.4.x

[Jun Omae]

It is unable to import trac.db_default before trac.db.api due to circular imports on Trac 1.4.x. It doesn't occur on Trac 1.5.x.

I noticed it while investigating ​th:#14054.

$ ~/venv/trac/1.4.3/bin/python
Python 2.7.18 (default, Mar  8 2021, 13:02:45)
[GCC 9.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from trac import __version__
>>> __version__
'1.4.3'
>>> from trac import db_default
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/jun66j5/venv/trac/1.4.3/local/lib/python2.7/site-packages/trac/db_default.py", line 17, in <module>
    from trac.db.schema import Table, Column, Index
  File "/home/jun66j5/venv/trac/1.4.3/local/lib/python2.7/site-packages/trac/db/__init__.py", line 14, in <module>
    from trac.db.api import *
  File "/home/jun66j5/venv/trac/1.4.3/local/lib/python2.7/site-packages/trac/db/api.py", line 23, in <module>
    from trac import db_default
ImportError: cannot import name db_default
>>> 
>>> import trac.db.api
>>> from trac import db_default

[Ryan J Ollos]

Milestone renamed