email.header: MicroPython doesn't have str.isspace().

And the usecase here clear doesn't prompt for it to be added to core language.

email.header/email/header.py

@@ -312,7 +312,7 @@ class Header:
     def _nonctext(self, s):
         """True if string s is not a ctext character of RFC822.
         """
-        return s.isspace() or s in ('(', ')', '\\')
+        return s in (' ', '\t', '(', ')', '\\')
 
     def encode(self, splitchars=';, \t', maxlinelen=None, linesep='\n'):
         r"""Encode a message header into an RFC-compliant format.

email.header/metadata.txt

@@ -1,4 +1,4 @@
 srctype = cpython
 type = package
-version = 0.5
+version = 0.5.1
 depends = re-pcre, binascii, email.encoders, email.errors, email.charset

email.header/setup.py

@@ -6,7 +6,7 @@ from setuptools import setup
 
 
 setup(name='micropython-email.header',
-      version='0.5',
+      version='0.5.1',
       description='CPython email.header module ported to MicroPython',
       long_description='This is a module ported from CPython standard library to be compatible with\nMicroPython interpreter. Usually, this means applying small patches for\nfeatures not supported (yet, or at all) in MicroPython. Sometimes, heavier\nchanges are required. Note that CPython modules are written with availability\nof vast resources in mind, and may not work for MicroPython ports with\nlimited heap. If you are affected by such a case, please help reimplement\nthe module from scratch.',
       url='https://github.com/micropython/micropython/issues/405',