Undefined names: import ParseError, ValidationError for line 85

[cclauss]

ParseError and ValidationError are both used on line 85 but they are not defined or imported.

flake8 testing of https://github.com/encode/apistar on Python 3.9.0rc2+

$ flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics

./apistar/cli.py:85:17: F821 undefined name 'ParseError'
        except (ParseError, ValidationError) as exc:
                ^
./apistar/cli.py:85:29: F821 undefined name 'ValidationError'
        except (ParseError, ValidationError) as exc:
                            ^
./apistar/schemas/autodetermine.py:15:20: F821 undefined name 'OPEN_API'
            return OPEN_API.validate(value, strict=strict)
                   ^
./apistar/schemas/autodetermine.py:17:20: F821 undefined name 'SWAGGER'
            return SWAGGER.validate(value, strict=strict)
                   ^
4     F821 undefined name 'ParseError'
4

https://flake8.pycqa.org/en/latest/user/error-codes.html

On the flake8 test selection, this PR does not focus on "style violations" (the majority of flake8 error codes that psf/black can autocorrect). Instead, these tests are focus on runtime safety and correctness:

• E9 tests are about Python syntax errors usually raised because flake8 can not build an Abstract Syntax Tree (AST). Often these issues are a sign of unused code or code that has not been ported to Python 3. These would be compile-time errors in a compiled language but in a dynamic language like Python, they result in the script halting/crashing on the user.
• F63 tests are usually about the confusion between identity and equality in Python. Use ==/!= to compare str, bytes, and int literals is the classic case. These are areas where a == b is True but a is b is False (or vice versa). Python >= 3.8 will raise SyntaxWarnings on these instances.
• F7 tests logic errors and syntax errors in type hints
• F82 tests are almost always undefined names which are usually a sign of a typo, missing imports, or code that has not been ported to Python 3. These also would be compile-time errors in a compiled language but in Python, a NameError is raised which will halt/crash the script on the user.