lang/python3[89]: Fix build WITH_DEBUG

Python 3.8 [0] and 3.9 [1] are using a value for "EXT_SUFFIX"
that includes "SOABI" on FreeBSD, C extension module filenames
are affected by this change [2].

[0]: https://github.com/python/cpython/commit/b01091a3e71e6636d2df4db45920e820cdf7df3b
[1]: https://github.com/python/cpython/commit/4b155967b3e743cbdc31600f13f1bfcf07f7b6ce
[2]: https://github.com/python/cpython/blob/v3.8.11/Lib/distutils/command/build_ext.py#L675-683

PR: 256558
Approved by: koobs (python, maintainer)
MFH: 2020Q3 (blanket: bug & regression fix)