Fix race condition in MkdirFunc

[vit9696]

With variant dirs SCons creates directories in the build directory per each SCons file. And this can race when parallel SCons tasks are launched in the same directory even if they have separate targets and sconsign files.

The issue code in question is as follows:

• _SConscript performs directory creation.
• It calls Dir's _create() method
• and that one reaches MkdirFunc through MkdirBuilder (SCons.Node.Node.build())

MkdirFunc function already checks for t.exists(), so as discussed on the chat a workaround to make this function atomic should be more or less ok.

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 read the README.rst)
• I have updated the appropriate documentation

No documentation or test changes should be necessary for this commit.

CC @mwichmann

[mwichmann]

Note the one Windows test suite failure is unrelated to the change, it's a different race, which #4402 was just merged to help with - though that came too late for this build.

[00:21:32] JSONDecodeError: Expecting value: line 1 column 1 (char 0):
[00:21:32]   File "C:\Users\appveyor\AppData\Local\Temp\1\testcmd.1860.nq3xr9bv\SConstruct", line 3:
[00:21:32]     env = Environment(MSVC_VERSION='14.3', MSVC_TOOLSET_VERSION='14.3', tools=['msvc'])
[00:21:32]   File "C:\projects\scons\SCons\Environment.py", line 1244:

[mwichmann]

As mentioned before this became a PR, this looks simple and straightforward to me.

[bdbaddog]

Looks good to me.
Can you add same blurb from CHANGES.txt to RELEASE.txt?

Also, entries in CHANGES.txt are generally sorted per release by the contributors last name.
Can you move your update to CHANGES.txt to be ordered as such and not at the end of the changes for the current (working) release?

Thanks for this fix!

[vit9696]

Done. Also rebased and squashed to fix type annotation for the function header.