mkdocs build will fail when as_datetime is true

[hitsumabushi845]

Hi
I'm facing the problem below, please let me know if there is some workaround to build document with as_datetime setting is enabled.

Description

When if as_datetime: true as the plugin setting, then the command mkdocs build will fail by the following error:

ERROR   -  Error reading page 'index.md': decoding to str: need a bytes-like object, datetime.datetime found 
Traceback (most recent call last):
  File "/usr/local/bin/mkdocs", line 8, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/mkdocs/__main__.py", line 152, in build_command
    build.build(config.load_config(**kwargs), dirty=not clean)
  File "/usr/local/lib/python3.8/site-packages/mkdocs/commands/build.py", line 271, in build
    _populate_page(file.page, config, files, dirty)
  File "/usr/local/lib/python3.8/site-packages/mkdocs/commands/build.py", line 167, in _populate_page
    page.markdown = config['plugins'].run_event(
  File "/usr/local/lib/python3.8/site-packages/mkdocs/plugins.py", line 94, in run_event
    result = method(item, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/mkdocs_git_revision_date_plugin/plugin.py", line 57, in on_page_markdown
    markdown = re.sub(r"\{\{(\s)*git_revision_date(\s)*\}\}",
  File "/usr/local/lib/python3.8/re.py", line 210, in sub
    return _compile(pattern, flags).sub(repl, string, count)
  File "/usr/local/lib/python3.8/re.py", line 327, in _subx
    template = _compile_repl(template, pattern)
  File "/usr/local/lib/python3.8/re.py", line 318, in _compile_repl
    return sre_parse.parse_template(repl, pattern)
  File "/usr/local/lib/python3.8/sre_parse.py", line 972, in parse_template
    s = Tokenizer(source)
  File "/usr/local/lib/python3.8/sre_parse.py", line 228, in __init__
    string = str(string, 'latin1')
TypeError: decoding to str: need a bytes-like object, datetime.datetime found

Steps to reproduce the bug

The problem can be reproduced by using the simple project below with the package versions below:
https://github.com/hitsumabushi845/git-revision-issue-reproduction

Package versions

• Python: 3.8.6
• MkDocs: 1.1.2
• git-revision-plugin: 0.3.1

[hitsumabushi845]

I'm investigating this issue.
it is caused by regex replacing as reported in the stacktrace above:

  File "/usr/local/lib/python3.8/site-packages/mkdocs_git_revision_date_plugin/plugin.py", line 57, in on_page_markdown
    markdown = re.sub(r"\{\{(\s)*git_revision_date(\s)*\}\}",

The repl argument of re.sub should be string or function but it is taking datetime object because of as_datetime: true.

This change is created by the PR #10, and it had been provided to solve the issue #9.

I have no idea to avoid both (reported here and #9) issues for now...I'll investigate for more.