requirements: update graphviz requirement from <0.20.0 to <0.21.0

[dependabot]

Updates the requirements on graphviz to permit the latest version.

Changelog

Sourced from graphviz's changelog.

Version 0.20

Add keyword-only neato_no_op argument to .render(), .pipe(), and stand-alone graphviz.render() and graphviz.pipe().

When building a Graph or Digraph, warn about an expected DOT syntax error in rendering when passing a string that ends with an odd number of backslashes (e.g. invalid dot.node('spam', label='\\') instead of correct ..., label=r'\\' for a node labled as a backslash).

Increase visibility of graphviz.escape() in the documentation.

Version 0.19.2

Drop Python 3.6 support (end of life 23 Dec 2021).

Fix ExecutableNotFound and CalledProcessError in graphviz.__all__.

Better document 0.18 change of behaviour for the body argument/attribute (lines need to include their final newline).

Version 0.19.1

Fix undecoded CalledProcessError.stdout and .stderr when .pipe() call with an encoding different from self.encoding fails.

Fix missing project root conftest.py in source distribution.

Extend examples/graphviz-escapes.ipynb.

Improve test coverage.

Increase build scripts verbosity.

Version 0.19

Add PendingDeprecationWarning to calls using positional arguments that will be deprecated in a later version. The future API will allow from one to three positional arguments depending on the method or function. Keyword-only arguments where not around when this library was created.

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Commits

• cbf6638 release 0.20
• 5ac24c2 improve type annotations
• c249d31 document latest documentation change
• 25043ed better document caveats on quoting/escaping
• 9a63a50 expose/document DotSyntaxWarning, standardize exception order
• fc75da8 render example notebooks with development version on binder
• 68e69c7 warn with DotSyntaxWarning when quoting strings ending in an odd number of ba...
• 517f1a4 bump minimal sphinx dependency in docs extras_requires to 4
• 9326bde use latest Python 3 version for building docs
• 617a9be group xlib and x11 following https://graphviz.org/docs/outputs
• Additional commits viewable in compare view

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[github-actions]

This PR causes the following changes to the html docs (ubuntu-latest-3.7-x64):

No differences!

...

See CI logs for the full diff.

[jvesely]

@dependabot squash and merge