Here is repro steps:
1. git clone -b "3.6" -c core.autocrlf=true https://github.com/python/cpython.git F:\gitP\python\cpython
2. Open a VS 2019 16.4.5 x86 command prompt and browse to F:\gitP\python\cpython
3. checkout the revision to f3a5b7a.
4. Add #include <windows.h> to the _iomodule.c.
5. cd F:\gitP\python\cpython\PCBuild
6. devenv /upgrade pcbuild.sln
7. build -e -r --no-tkinter -v "/p:PlatformToolset=v142" "/p:WindowsTargetPlatformVersion=10.0.18362.0" 
8.cd F:\gitP\python\cpython\PCBuild
9.rt -q -x test_math test_cmath test_distutils test_codecencodings_iso2022 test_random test_sax test_enum test_re test_subprocess test_socket
Error:
4 tests failed:
    test_binhex test_importlib test_peg_generator test_tools
Please see the attachment for details

Runtest itself ran to completion but the 4 tests listed failed.  I suspect that this should be closed as "won't fix" because
A. 3.6 only gets security fixes since Dec 2018.
B. Locally built Windows binaries often fail a few tests, either occasionally or dependably even if the Windows buildbots are green.  Just now, test_import (still, or again), test__locale, and test_locale failed for me with 3.9.0a6+.  This is unfortunate, but it is sometimes not clear whether the bug is in the modules or the tests.

Thanks for the report.  Unfortunately, you are not actually building Python 3.6 here.  Revision f3a5b7a is on the master branch, not 3.6. The fact that you are seeing a failure of test_peg_generator confirms that you are building from master (i.e. a pre-release of 3.9), since that test and the feature behind it are new in 3.9.

P.S. If you *do* want to build the current HEAD of the 3.6 branch including as yet unreleased fixes:

git checkout 3.6

If you want to build a specific release, use the v^ tags provided for each release:

git checkout v3.6.8