Correctly validate evaluation expression input

[PPakalns]

• There must be exactly one statement in input
• Given statement must be an expression (assignment, augmented assignment, etc. are not allowed)

Resolves #58 #65 #67

[pep8speaks]

Hello @PPakalns! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2019-12-12 10:30:39 UTC

[coveralls]

Coverage increased (+2.6%) to 99.602% when pulling 7ef2214 on PPakalns:master into 66b04c9 on danthedeckie:master.

[coveralls]

Coverage increased (+1.7%) to 98.69% when pulling e3b4ab5 on PPakalns:master into 66b04c9 on danthedeckie:master.

[PPakalns]

I believe that these issues found by CodeFactor can be fixed in other pull request as I did not introduce them or I used the same style that was already used by this library.
If you do not agree I can of course fix these reported errors.

[jeffreyscottgraham]

Is this issue dead?

[PPakalns]

@jeffreyscottgraham
There are users that are already dependent on the way how this library is implemented. See this comment and the whole thread.

Looks like 2.0 version of this library needs to be created. If someone needs additional functionality, then new fork of this library, probably, must be created because the activity on this project is very low.

[kurtmckee]

@danthedeckie I recommend closing this PR due to its age.