import re

mystring = 'this is my first sentence with (brackets)in it. this is the second?What about this sentence with D.D.T. in it?or this with 4.5?'

def fix_punctuation(text):
    def sentence_case(text):
        # Split into sentences. Therefore, find all text that ends
        # with punctuation followed by white space or end of string.
        sentences = re.findall(r'(?:\d+\.\d+|\b[A-Z](?:\.[A-Z])*\b\.?|[^.!?])+[.!?](?:\s|\Z)', text)

        # Capitalize the first letter of each sentence
        sentences = [x[0].upper() + x[1:] for x in sentences]

        # Combine sentences
        return ''.join(sentences)
    
    #add space after punctuation
    text = re.sub(r'(\d+\.\d+|\b[A-Z](?:\.[A-Z])*\b\.?)|([.,;:!?)])\s*', lambda x: x.group(1) or f'{x.group(2)} ', text)
    #capitalize sentences
    return sentence_case(text)
    
print(fix_punctuation(mystring))