Make improvements to the "Deferred Assert" system

Make improvements to the "Deferred Assert" system

SeleniumBase deferred asserts allow you to make multiple assertions on the same page without failing the test after the first failed assert. Instead, you can choose when to process those assertions by calling: self.process_deferred_asserts().

• Add self.deferred_assert_exact_text()
  -- (This is similar to self.deferred_assert_text(), but text must be equal, instead of in.)
• Fix spacing in the console output of deferred assertion failures.
• This resolves #1065
• Example test: SeleniumBase/examples/test_deferred_asserts.py

Also refresh Python dependencies:

• traitlets version now only has a lower bound, instead of an exact version.
  -- (For Python versions 3.7 and newer. Earlier Python versions still use a pin.)

traitlets==4.3.3;python_version<"3.7"
traitlets>=5.1.1;python_version>="3.7"