Prepare for 1.1.1

[RobertoPrevato]

• Improve the contribs plugin to not pollute the logs with
  dateutil.parser.ParseError while working on a new file that is not
  committed in Git, yet.
• Add the possibility to enable and disable the contribs plugin by env
  variable, through plugin configuration. To use, specify the following
  setting:

  - neoteroi.contribs:
      enabled_by_env: "GIT_CONTRIBS_ON"  # Use the name you wish here for the env var

• When enabled_by_env is not empty, the Git contributors plugin is only
  enabled if such variable exists and its value is a value in {"1", "true"}
  (case insensitive). This is useful to disable the plugin by default during
  local development, and enable it only during an automated build that builds
  the documentation for publishing. The rationale for this setting is that
  this plugin has an heavy impact on the build performance as it uses the Git
  CLI to obtain the list of contributors who worked on each page.
• Remove Python 3.8 from the build matrix, add 3.13.