fix: use "==" with literal to remove warning during "python setup.py updateplotlyjs"

[gvwilson]

• Existing code produces this warning during python setup.py updateplotlyjs step from contributing.md:

/Users/gvwilson/plotly.py/packages/python/chart-studio/chart_studio/tools.py:290: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if share_key is "":

• Revised code uses == instead of is

  • Warning goes away
  • No change in test status

• [X] I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.