BUG: Python parser breaks with quotes and multi-char sep 
On `master` (<a href="https://github.com/pydata/pandas/commit/b722222f5ea760a3f3df4d063309949eb4956674">b722222</a>):

``` python
>>> data = 'a,,b\n1,,a\n2,,"2,,b"'
>>> read_csv(StringIO(data), sep=',,', engine='python')
...
ValueError: Expected 2 fields in line 3, saw 3
```

I expect this command to work, but because no parsing is done on quoted fields as can be seen <a href="https://github.com/pydata/pandas/blob/master/pandas/io/parsers.py#L1847">here</a>, an extra field is produced, breaking the parser.  Note that this does not affect the C parser because multi-char delimiters are not supported.  Similar to what we saw in #10911 and #12775, but unless we want to write the `tokenizer.c` code in Python, a similar fix does not seem trivial.
