Update versioneer.py to 0.21

[eclipseo]

This is needed for Python 3.11:

Traceback (most recent call last):
  File "/builddir/build/BUILD/constantly-15.1.0/setup.py", line 41, in <module>
    version=versioneer.get_version(),
            ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/builddir/build/BUILD/constantly-15.1.0/versioneer.py", line 1405, in get_version
    return get_versions()["version"]
           ^^^^^^^^^^^^^^
  File "/builddir/build/BUILD/constantly-15.1.0/versioneer.py", line 1339, in get_versions
    cfg = get_config_from_root(root)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/builddir/build/BUILD/constantly-15.1.0/versioneer.py", line 399, in get_config_from_root
    parser = configparser.SafeConfigParser()
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: module 'configparser' has no attribute 'SafeConfigParser'. Did you mean: 'RawConfigParser'?

Removed from the configparser module: the SafeConfigParser class, the filename
property of the ParsingError class, the readfp() method of the ConfigParser
class, deprecated since Python 3.2. (Contributed by Hugo van Kemenade in
bpo-45173.)
https://docs.python.org/3.11/whatsnew/3.11.html

WARNING: latest versioneer is only compatible with Python 3.x

[bkuhls]

ping, please apply this commit and consider a new release, thanks!

[graingert]

Thanks for the PR, this was resolved in #35