Fix #4087 by setting the correct csv dialect (#4103)

The error in #4087 was happening because of the use of csv.Dialect.*
which is just an empty base class. we need to make a choice on what is
our base dialect. I usually use excel so I put it as excel, if
maintainers have other preferences do let me know.

Open Questions:
1. What should be the default dialect?
2. Should we rework all tests to mock the open function rather than the
csv.DictReader?
3. Should we make a separate input for `dialect` like we have for
`encoding`?

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Co-authored-by: = <=>

tests/unit_tests/document_loader/test_docs/csv/test_nominal.csv

@@ -0,0 +1,3 @@
+column1,column2,column3
+value1,value2,value3
+value4,value5,value6

tests/unit_tests/document_loader/test_docs/csv/test_one_col.csv

@@ -0,0 +1,4 @@
+column1
+value1
+value2
+value3

tests/unit_tests/document_loader/test_docs/csv/test_one_row.csv

@@ -0,0 +1,2 @@
+column1,column2,column3
+value1,value2,value3