Add support for CVSSv4 (#45)

* Add support for CVSSv4

-Added class CVSS4
-Based the CVSS4 class on the js implementation found here:
https://github.com/RedHatProductSecurity/cvss-v4-calculator
-Added new tests for CVSS4
-Added new constants and exceptions for CVSS4
-Added json schema for CVSS4

* CVSSv4, skontar review changes

-Fixed various typos: CVSS3 -> CVSS4
-Added malformed CVSS4MalformedError check for wrong cvss prefix
-Fixed prefix in clean_vector: "CVSS:4/" -> "CVSS:4.0/"

* CVSSv4, mprpic review changes

-Added additional CVSS4 imports to cvss/__init__.py
-Added additional metric checks that raise CVSS4MalformedError
-Cleaned up some left over comments
-Added additional tests for malformed CVSS4 strings

* cvss4, Removed duplicate Decimal import

cvss/__init__.py

@@ -4,7 +4,8 @@
 
 from .cvss2 import CVSS2
 from .cvss3 import CVSS3
-from .exceptions import CVSS2Error, CVSS3Error, CVSSError
+from .cvss4 import CVSS4
+from .exceptions import CVSS2Error, CVSS3Error, CVSS4Error, CVSSError
 from .interactive import ask_interactively
 
 __version__ = "2.6"