Write a Python script that uses the Twitter API to fetch tweets containing specific keywords or hashtags, processes the tweet text (e.g., removing URLs and special characters), and performs sentiment analysis on the processed text using a machine learning library like scikit-learn.

To accomplish this task, you'll need to use the Tweepy library for accessing the Twitter API and TextBlob or scikit-learn for sentiment analysis. Here's a step-by-step guide on how to do it:

1. Install required libraries:
```bash
pip install tweepy textblob
```

2. Create a new Python script and import necessary modules:
```python
import tweepy
from textblob import TextBlob
import re
```

3. Set up your Twitter API credentials (you can get these from https://developer.twitter.com/):
```python
consumer_key = 'your_consumer_key'
consumer_secret = 'your_consumer_secret'
access_token = 'your_access_token'
access_token_secret = 'your_access_token_secret'
```

4. Authenticate with the Twitter API using Tweepy:
```python
auth = tweepy.OAuthHandler(consumer_key, consumer_secret)
auth.set_access_token(access_token, access_token_secret)

api = tweepy.API(auth)
```

5. Define a function to clean tweet text by removing URLs and special characters:
```python
def clean_tweet_text(tweet):
    return ' '.join(re.sub("(@[A-Za-z0-9]+)|([^0-9A-Za-z \t])|(\w+:\/\/\S+)", " ", tweet).split())
```

6. Define a function to perform sentiment analysis using TextBlob:
```python
def analyze_sentiment(tweet):
    analysis = TextBlob(clean_tweet_text(tweet))
    
    if analysis.sentiment.polarity > 0:
        return 'Positive'
    elif analysis.sentiment.polarity == 0:
        return 'Neutral'
    else:
        return 'Negative'
```

7. Fetch tweets containing specific keywords or hashtags and perform sentiment analysis:
```python
search_term = 'your_search_term_here'
num_of_tweets = 100

tweets = tweepy.Cursor(api.search, q=search_term, lang='en').items(num_of_tweets)

for tweet in tweets:
    print(f"{tweet.text} -> {analyze_sentiment(tweet.text)}")
```

Replace 'your_search_term_here' with the keyword or hashtag you want to search for. This script will fetch 100 tweets containing the specified term and perform sentiment analysis on each of them.

Please note that this example uses TextBlob for sentiment analysis, which is a simple library and may not provide highly accurate results. For more advanced sentiment analysis, consider using scikit-learn or other machine learning libraries.