BUG: Fix for to_excel +/- infinity
BUG: Previously, a negative sign was being prepended for positive infinity, not for negative infinity. (GH6812)

xref #6812 

- need a tests for this (write a frame with both pos/neg inf, read back and check that they match)
  should fail w/o the fix and pass after
- need a release note in v0.15.0.txt (bug fix section), reference this issue number
 status of this? (did you see my comments above)?
 There was an existing test (`test_inf_roundtrip()` in test_excel.py) that wrote/read a frame with positive and negative infinity, but it was not catching the error due to an oversight in the underlying comparison. In testing.pyx, `assert_almost_equal()` was only looking for infinite values, regardless of +/-. Adding the check for +/- there allows `test_inf_roundtrip()` to work as expected. 
 @maxchang ok, makes sense.

pls rebase and squash and then I think good to go
 @jreback Ok, should be ready now.
 thanks!
