A few more small tweaks to Environment

[mwichmann]

Fix a couple of small typing problems, fold some really long signature lines, improve a few docstrings, and perform a few more lint-type complaints.

There's no intentional behavior change, tests seem to confirm that, but let's wait for a CI run.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation

[mwichmann]

Question because some of the changes were around SConsignFile() anyway:

If name is set to None, scons will store file signatures in a separate .sconsign file in each directory, not in a single combined database file. This is a backwards-compatibility measure to support what was the default behavior prior to SCons 0.97 (i.e. before 2008). Use of this mode is discouraged and may be deprecated in a future SCons release.

Did we want to act further on that? Transition from "discouraged" to "deprecated"?

[Repiteo]

Did we want to act further on that? Transition from "discouraged" to "deprecated"?

I'd prefer to go to "deprecated", personally. Despite how big this project is on maintaining backwards compatibility, trying to extend that to pre-1.0 releases is excessive