The last GCC 3.x release before 4.0 was 3.4 which was released 2004,
so 17 years ago.

Removes all code only used with those old toolchains.

The comment in the original code came about because of the bug fixed in
gh-20155.  The fixed subsequent clause should now give the correct
assembly version for the given Python.

In fact, the original clause matches the MSVC for Python 3.7.1:

```
PS C:\> .\Python37\python.exe .\io\check_versions.py
sys.version 3.7.1 (v3.7.1:260ec2c36a, Oct 20 2018, 14:57:15) [MSC v.1915 64 bit (AMD64)]
distutils build version 14.1
msvcrt.CRT_ASSEMBLY_VERSION 14.15.26726.0
```

Original algorithm appended both character of minor version > 9, but the
string against which it is being matched, only uses the first character.

The float representation of e.g. 14.2 is not exact, leading to a minor
version number of int(1.999999) == 1 in the original code.

File may not be closed if an exception is raised.

* MAINT: use list-based APIs to call subprocesses
* TST, MAINT: add a test for mingw32ccompiler.build_import, clean up lib2def
Co-authored-by: Matti Picus <matti.picus@gmail.com>
Co-authored-by: Eric Wieser <wieser.eric@gmail.com>

* Cleanup unused imports (F401) of mostly standard Python modules,
  or some internal but unlikely referenced modules
* Where internal imports are potentially used, mark with noqa
* Avoid redefinition of imports (F811)