Use newline='' for csv.writer

Fixes saulpw#1362

According to the docs for csv.writer:

> If csvfile is a file object, it should be opened with newline=''
> https://docs.python.org/3/library/csv.html#csv.writer

Additionally:

> If newline='' is not specified, newlines embedded inside quoted fields will
> not be interpreted correctly, and on platforms that use \r\n linendings on
> write an extra \r will be added. It should always be safe to specify
> newline='', since the csv module does its own (universal) newline handling.
> https://docs.python.org/3/library/csv.html#id3

This commit adds `newline=None` as a keyword argument for `open_text` in
`path.py`. It then uses this new keyword argument when opening a file
for writing in `loaders/csv.py`

visidata/loaders/csv.py

@@ -47,7 +47,7 @@ def iterload(self):
 @VisiData.api
 def save_csv(vd, p, sheet):
     'Save as single CSV file, handling column names as first line.'
-    with p.open_text(mode='w', encoding=sheet.options.encoding) as fp:
+    with p.open_text(mode='w', encoding=sheet.options.encoding, newline='') as fp:
         cw = csv.writer(fp, **options.getall('csv_'))
         colnames = [col.name for col in sheet.visibleCols]
         if ''.join(colnames):

visidata/path.py

@@ -159,7 +159,7 @@ def __lt__(self, a):
     def __truediv__(self, a):
         return Path(self._path.__truediv__(a))
 
-    def open_text(self, mode='rt', encoding=None):
+    def open_text(self, mode='rt', encoding=None, newline=None):
         'Open path in text mode, using options.encoding and options.encoding_errors.  Return open file-pointer or file-pointer-like.'
         # rfile makes a single-access fp reusable
 
@@ -183,7 +183,7 @@ def open_text(self, mode='rt', encoding=None):
                 vd.error('invalid mode "%s" for Path.open_text()' % mode)
                 return sys.stderr
 
-        return self.open(mode=mode, encoding=encoding or vd.options.encoding, errors=vd.options.encoding_errors)
+        return self.open(mode=mode, encoding=encoding or vd.options.encoding, errors=vd.options.encoding_errors, newline=newline)
 
     @wraps(pathlib.Path.read_text)
     def read_text(self, *args, **kwargs):