Request text mode to Popen to avoid buffering warning (python 3.8)

Python 3.8 started reporting warnings when binary files are opened with
bufsize > 0, see https://bugs.python.org/issue32236 for the change and
benoitc/gunicorn#2091 for a similar issue.

The error message was:

py.warnings: WARNING: /usr/lib/python3.8/subprocess.py:844: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used

conans/client/conf/detect.py

@@ -10,7 +10,8 @@
 
 
 def _execute(command):
-    proc = Popen(command, shell=True, bufsize=1, stdout=PIPE, stderr=STDOUT)
+    proc = Popen(command, shell=True, bufsize=1, universal_newlines=True, stdout=PIPE,
+                 stderr=STDOUT)
 
     output_buffer = []
     while True: