Fix some unused import lint warnings

import2ledger/importers/amazon.py

@@ -1,4 +1,3 @@
-import datetime
 import itertools
 
 from . import _csv

import2ledger/importers/github.py

@@ -1,5 +1,3 @@
-import decimal
-
 from . import _csv
 from .. import strparse
 

import2ledger/importers/nbpy2017.py

@@ -139,6 +139,7 @@ def _parse_invoice(parser_class, source_file):
     except AttributeError:
         return None
 
+
 class InvoiceImporter:
     INVOICE_CLASS = Invoice2017
     LEDGER_TEMPLATE_KEY_FMT = 'nbpy2017 {0} ledger entry'

import2ledger/importers/patreon.py

@@ -132,7 +132,6 @@ class VATImporter(FeeImporterBase):
     }
 
 
-
 class Income2020OctoberImporter(IncomeImporter):
     AMOUNT_KEY = 'Pledge $'