black: make codebase compatible with black v21.4b2 and v20.8b1 I don't know what exact version of black made it care about these whitespace differences, but this is the version I got when I just installed it with `pip3 install black`. I'm intentionally not increasing the version of black required, as I don't want to force everyone to upgrade their version of black, and these fixes are backwards compatible with black v20.8b1. If there are more issues in the future and this becomes a maintenance burden I may do so in a future change. Tested with both versions of black (I got the older version via `pip3 install black==20.8b1`) Differential Revision: https://phab.mercurial-scm.org/D10539

--- a/mercurial/mail.py	Tue May 04 13:28:42 2021 -0400
+++ b/mercurial/mail.py	Fri Apr 30 16:13:02 2021 -0700
@@ -269,7 +269,7 @@
 
 def codec2iana(cs):
     # type: (str) -> str
-    ''''''
+    ''' '''
     cs = email.charset.Charset(cs).input_charset.lower()
 
     # "latin1" normalizes to "iso8859-1", standard calls for "iso-8859-1"