all: remove explicit versions in USES=python for "3.x+"

The logic in USES=python will automatically convert this to 3.8+ by
itself.

Adjust two ports that only had Python 3.7 mentioned but build fine
on Python 3.8 too.

finance/quickfix: mark BROKEN with PYTHON

libtool: compile: c++ -DHAVE_CONFIG_H -I. -I../.. -I -I. -I.. -I../.. -I../C++ -DLIBICONV_PLUG -DPYTHON_MAJOR_VERSION=3 -Wno-unused-variable -Wno-maybe-uninitialized -O2 -pipe -DLIBICONV_PLUG -fstack-protector-strong -fno-strict-aliasing -DLIBICONV_PLUG -Wall -ansi -Wno-unused-command-line-argument -Wpointer-arith -Wwrite-strings -Wno-overloaded-virtual -Wno-deprecated-declarations -Wno-deprecated -std=c++0x -MT _quickfix_la-QuickfixPython.lo -MD -MP -MF .deps/_quickfix_la-QuickfixPython.Tpo -c QuickfixPython.cpp -fPIC -DPIC -o .libs/_quickfix_la-QuickfixPython.o
warning: unknown warning option '-Wno-maybe-uninitialized'; did you mean '-Wno-uninitialized'? [-Wunknown-warning-option]
QuickfixPython.cpp:175:11: fatal error: 'Python.h' file not found

^~~~~~~~~~

1 warning and 1 error generated.

Reviewed by: portmgr, vishwin, yuri
Differential Revision: https://reviews.freebsd.org/D40568