[3.6] bpo-30529: Fix errors for invalid whitespaces in f-string subexpressions. ...

[3.6] bpo-30529: Fix errors for invalid whitespaces in f-string subexpressions. (GH-1888) (#2013)

'invalid character in identifier' now is raised instead of
'f-string: empty expression not allowed' if a subexpression contains
only whitespaces and they are not accepted by Python parser.
(cherry picked from commit 2e9cd58)