Accept datetime instances as dates

[mildsunrise]

There's no easy way to re-create a commit (i.e. for rewriting purposes), because dates must be formatted as strings, passed, then parsed back.

This patch allows parse_date() to accept datetime instances, such as those produced by from_timestamp().
Re-creating a commit is then possible:

Commit.create_from_tree(
  repo=commit.repo,
  tree=commit.tree,
  message=commit.message,
  parent_commits=commit.parents,
  author=commit.author,
  author_date=commit.authored_datetime,
  committer=commit.committer,
  commit_date=commit.committed_datetime,
)

[Byron]

Thanks a lot for your contribution.
It looks great 👍!