Show error message when keywords expression contains a syntax error

[nawatts]

Related to #492.

Currently, providing a --keywords argument that is not a valid expression causes Nox to crash with a SyntaxError.

$ nox -k 'foo:bar'
Traceback (most recent call last):
  ...
  File "<string>", line 1
    foo:bar
       ^
SyntaxError: invalid syntax

This change validates the keywords argument using ast.parse and prints an error message if it is not a valid expression.

$ nox -k 'foo:bar'
nox > Error while collecting sessions: keywords argument must be a Python expression.

[FollowTheProcess]

Hi @nawatts, thanks for the contribution!

This looks good to me 👍🏻

[cjolowicz]

Thanks @nawatts

Just waiting for the CI to pass after the merge.