Revert "TST/TEMP: fix pyqt to 4.x for plotting tests"
Reverts pydata/pandas#14240

For when https://github.com/ContinuumIO/anaconda-issues/issues/1068 is solved

Also needs to revert https://github.com/pydata/pandas/commit/b99cb7ab225f432da4dfe73e0882e1de3db121b6
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14243?src=pr) is 85.26% (diff: 97.14%)

> Merging [#14243](https://codecov.io/gh/pydata/pandas/pull/14243?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **0.01%**

``` diff
@@             master     #14243   diff @@
==========================================
  Files           140        140          
  Lines         50559      50619    +60   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43099      43160    +61   
+ Misses         7460       7459     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [e8357a1...b99cb7a](https://codecov.io/gh/pydata/pandas/compare/e8357a15cd61ff698cbd3d57904133c586a8ed8b...b99cb7ab225f432da4dfe73e0882e1de3db121b6?src=pr)
 @jorisvandenbossche ? Wanted to close and reopen to trigger travis (to see if it is fixed), but can't reopen -> new PR https://github.com/pandas-dev/pandas/pull/14744