Use packaging to parse versions (#8314)

packaging is the standard tool for dealing with versions and
requirements. This allows us to easily support all PEP 508 specs.
packaging also has good error messages, so I don't create our own here.
This lets us get rid of our handrolled half parser and sanity checks.
I kept the whitespace consistency check, since that's just a formatting
lint and isn't testing the correctness of parsing (which we now
definitely do correctly).

CONTRIBUTING.md

@@ -80,16 +80,6 @@ terminal to install all non-pytype requirements:
 (.venv) > pip install -r requirements-tests.txt
 ```
 
-### Optional dependencies
-
-Several tests also have `termcolor` as an optional dependency. Installing this
-is not essential to run the tests, but can make the output of some of the tests
-slightly prettier and easier to read. To install `termcolor`, run:
-
-```
-pip install termcolor
-```
-
 ## Code formatting
 
 The code is formatted using `black` and `isort`.