Update to pytest 6.0

[danyeaw]

• Pytest 6.0 adds type support and initial support for configs in pyproject.toml.
• Move coveragerc config in to pyproject.toml
• Update dependencies
• Freeze usage of PyInstaller 3.6, the recently released 4.0 has some weird permission issue

PR Checklist

Please check if your PR fulfills the following requirements:

• I have read, and am following the Contributer guide
• I have read and understand the Code of Conduct

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Documentation content changes

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[sourcery-ai]

Sourcery Code Quality Report (beta)

❌  Merging this PR will decrease code quality in the affected files by 0.29 out of 10.

Quality metrics	Before	After	Change
Complexity	0.71	1.00	0.29 🔴
Method Length	34.53	45.54	11.01 🔴
Quality	9.08	8.79	-0.29 🔴

Other metrics	Before	After	Change
Lines	64	59	-5

Changed files	Quality Before	Quality After	Quality Change
gaphor/UML/states/pseudostates.py	8.80	8.79	-0.01 🔴

Please see our documentation here for details on how these metrics are calculated.

We are actively working on this report - lots more documentation and extra metrics to come!

Let us know what you think of it by mentioning @sourcery-ai in a comment.