API/BUG: Fix Series ops inconsistencies
- [x] closes #1134, closes #4581, closes #13538
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

This includes the fix only for #1134 and not for other inconsistencies (like #13637), as it needs further discussions.

Changes:
- series comparison operator to check whether labels are identical (currently: ignores labels)
- series boolean operator to align with labels (currently: only keeps left index)

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13894?src=pr) is 85.25% (diff: 100%)

> Merging [#13894](https://codecov.io/gh/pydata/pandas/pull/13894?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13894   diff @@
==========================================
  Files           139        139          
  Lines         50386      50394     +8   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42959      42965     +6   
- Misses         7427       7429     +2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [51b20de...fe322be](https://codecov.io/gh/pydata/pandas/compare/51b20de44fdcb998f25392558e029621e10c451c...fe322be77d77811bb27eadb6abb29dfe66c7bd63?src=pr)
 will have a look.

@jorisvandenbossche 
@wesm 
 For dataframes, the operator (eg `==`) is strict regarding indices, but the equivalent method (eg `df1.eq(df2)`) is flexible as it aligns. 
For series, both options raise now (current PR) or are broken (master). I think we should also make this distinction and make `s1.eq(s2)` flexible (=align and not raise) as discussed in https://github.com/pydata/pandas/issues/1134?
 This seems OK to me. I'm sure this will cause some API breakage but it would be best to encourage folks to use the flex comparison methods if they want auto-alignment
 In light of Wes' comment, I think the whatsnew notice can be a bit more explicit in that regard. So say that if you did `s1 == s2` before (which now raises), that you have to do `s1.values == s2.values` if you want to keep the same non-aligning / index-ignoring behaviour, or `s1.eq(d2)` to get auto-alignment.
 Modulo the doc comments, this is good to go for me!
 LGTM 
@jreback ?
 @sinhrks Thanks a lot!
