Fix warnings about invalid escape sequences

  /builds/pub/terrareg/terrareg/models.py:1469: DeprecationWarning: invalid escape sequence \{
    repo_clone_url_template = urllib.parse.quote(repo_clone_url_template, safe='\{\}/:@%?=')
terrareg/models.py:1517
  /builds/pub/terrareg/terrareg/models.py:1517: DeprecationWarning: invalid escape sequence \{
    repo_browse_url_template = urllib.parse.quote(repo_browse_url_template, safe='\{\}/:@%?=')
terrareg/models.py:1558
  /builds/pub/terrareg/terrareg/models.py:1558: DeprecationWarning: invalid escape sequence \{
    repo_base_url_template = urllib.parse.quote(repo_base_url_template, safe='\{\}/:@%?=')
../../../usr/local/lib/python3.8/dist-packages/requests/__init__.py:43