Even though it's 3 years later now, the disabling is possible with the -c flag.
pytesseract.image_to_string(roi, lang='eng', config='--psm 3 --oem 3 -c load_system_dawg=false -c load_freq_dawg=false -c 0tessedit_char_whitelist=0123456789r°:-abcdefghijklmnopqstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ')
From the tesseract.exe
-c VAR=VALUE Set value for config variables.
Multiple -c arguments are allowed.
--psm NUM Specify page segmentation mode.
--oem NUM Specify OCR Engine mode.
NOTE: These options must occur before any configfile.
Page segmentation modes:
0 Orientation and script detection (OSD) only.
1 Automatic page segmentation with OSD.
2 Automatic page segmentation, but no OSD, or OCR. (not implemented)
3 Fully automatic page segmentation, but no OSD. (Default)
4 Assume a single column of text of variable sizes.
5 Assume a single uniform block of vertically aligned text.
6 Assume a single uniform block of text.
7 Treat the image as a single text line.
8 Treat the image as a single word.
9 Treat the image as a single word in a circle.
10 Treat the image as a single character.
11 Sparse text. Find as much text as possible in no particular order.
12 Sparse text with OSD.
13 Raw line. Treat the image as a single text line,
bypassing hacks that are Tesseract-specific.
OCR Engine modes:
0 Legacy engine only.
1 Neural nets LSTM engine only.
2 Legacy + LSTM engines.
3 Default, based on what is available.