fix(config): fail on an invalid platform regex - #4104

An invalid platform value, such as platform = "linux(", made tox print an internal error with a traceback. A valid
expression that does not match skips the environment. A malformed one now fails it with a one-line error, as tox
already does for an invalid --skip-env pattern (#4029).

# before
$ tox r -e py
py: internal error
Traceback (most recent call last):
  ...
re.error: missing ), unterminated subpattern at position 5
  py: FAIL code 2

# after
$ tox r -e py
py: invalid platform regular expression 'linux(': missing ), unterminated subpattern at position 5
  py: FAIL code 1

The same applies when provisioning. An invalid platform on the .tox environment stops the run with one line:

ROOT: HandledError| invalid platform regular expression 'linux(': missing ), unterminated subpattern at position 5

Closes #4103.

• ran the linter to address style issues
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation (platform now notes that an invalid regex fails the environment)