modulefinder.py does not open source files in "rb" which
prevents compile() from applying the encoding rules.

This first showed up for me on Windows with Python 3.8.

Here is my test case and the results on Windows with 3.8.

import modulefinder

with open('import_functools.py', 'w') as f:
   f.write('import functools\n')

mf = modulefinder.ModuleFinder()
mf.run_script('import_functools.py')
print('Test passed')

py -3.8-32 modulefinder_test.py
Traceback (most recent call last):
 File "modulefinder_test.py", line 7, in <module>
   mf.run_script('import_functools.py')
 File "C:\Python38.win32\lib\modulefinder.py", line 165, in run_script
   self.load_module('__main__', fp, pathname, stuff)
 File "C:\Python38.win32\lib\modulefinder.py", line 360, in load_module
   self.scan_code(co, m)
 File "C:\Python38.win32\lib\modulefinder.py", line 433, in scan_code
   self._safe_import_hook(name, m, fromlist, level=0)
 File "C:\Python38.win32\lib\modulefinder.py", line 378, in _safe_import_hook
   self.import_hook(name, caller, level=level)
 File "C:\Python38.win32\lib\modulefinder.py", line 177, in import_hook
   q, tail = self.find_head_package(parent, name)
 File "C:\Python38.win32\lib\modulefinder.py", line 233, in find_head_package
   q = self.import_module(head, qname, parent)
 File "C:\Python38.win32\lib\modulefinder.py", line 326, in import_module
   m = self.load_module(fqname, fp, pathname, stuff)
 File "C:\Python38.win32\lib\modulefinder.py", line 343, in load_module
   co = compile(fp.read()+'\n', pathname, 'exec')
 File "C:\Python38.win32\lib\encodings\cp1252.py", line 23, in decode
   return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x81 in position 308: character maps to <undefined>

Barry