Fix linting

Need to escape the single slash in docstring regexes.
It is an error in Python 3.8

See PyCQA/pycodestyle#854 for

plydata/helper_verbs.py

@@ -1655,7 +1655,7 @@ class group_by_at(_at):
     group columns.
 
     >>> def double(s): return s + s
-    >>> df >> group_by_at(dict(matches=r'\w+eta$'), double)
+    >>> df >> group_by_at(dict(matches=r'\\w+eta$'), double)
     groups: ['beta', 'theta']
       alpha beta theta  x  y   z
     0     a   bb    cc  1  6   7

plydata/one_table_verbs.py

@@ -270,7 +270,7 @@ class select(DataOperator):
     0        1    1     1
     1        2    2     2
     2        3    3     3
-    >>> df >> select('bell',  matches='\w+tle$')
+    >>> df >> select('bell',  matches=r'\\w+tle$')
        bell  whistle
     0     1        1
     1     2        2

plydata/tests/test_dataframe.py

@@ -149,13 +149,13 @@ def test_select():
     assert len(result.columns) == 3
 
     result = df >> select('caracal', endswith='ar', contains='ee',
-                          matches='\w+opa')
+                          matches=r'\w+opa')
     assert len(result.columns) == 5
 
     result = df >> select(contains=['ee', 'ion', '23'])
     assert len(result.columns) == 2
 
-    result = df >> select(matches=('\w+opa', '\w+r$'))
+    result = df >> select(matches=(r'\w+opa', r'\w+r$'))
     assert len(result.columns) == 4
 
     # grouped on columns are never dropped
@@ -176,7 +176,7 @@ def test_select():
     df[123] = 1
     df[456] = 2
     df[789] = 3
-    pattern = re.compile('\w+opa')
+    pattern = re.compile(r'\w+opa')
     result = df >> select(startswith='t', matches=pattern)
     assert len(result.columns) == 2