Added function that checks if a string is an isogram
diff --git a/strings/is_isogram.py b/strings/is_isogram.py
new file mode 100644
index 000000000000..a9d9acc8138e
--- /dev/null
+++ b/strings/is_isogram.py
@@ -0,0 +1,30 @@
+"""
+wiki: https://en.wikipedia.org/wiki/Heterogram_(literature)#Isograms
+"""
+
+
+def is_isogram(string: str) -> bool:
+    """
+    An isogram is a word in which no letter is repeated.
+    Examples of isograms are uncopyrightable and ambidextrously.
+    >>> is_isogram('Uncopyrightable')
+    True
+    >>> is_isogram('allowance')
+    False
+    >>> is_isogram('copy1')
+    Traceback (most recent call last):
+     ...
+    ValueError: String must only contain alphabetic characters.
+    """
+    if not all(x.isalpha() for x in string):
+        raise ValueError("String must only contain alphabetic characters.")
+
+    letters = sorted(string.lower())
+    return len(letters) == len(set(letters))
+
+
+if __name__ == "__main__":
+    input_str = input("Enter a string ").strip()
+
+    isogram = is_isogram(input_str)
+    print(f"{input_str} is {'an' if isogram else 'not an'} isogram.")

### Describe your change:



* [x] Add an algorithm?
* [ ] Fix a bug or typo in an existing algorithm?
* [ ] Documentation change?

### Checklist:
* [x] I have read [CONTRIBUTING.md](https://github.com/TheAlgorithms/Python/blob/master/CONTRIBUTING.md).
* [x] This pull request is all my own work -- I have not plagiarized.
* [x] I know that pull requests will not be merged if they fail the automated tests.
* [ ] This PR only changes one algorithm file.  To ease review, please open separate PRs for separate algorithms.
* [x] All new Python files are placed inside an existing directory.
* [x] All filenames are in all lowercase characters with no spaces or dashes.
* [x] All functions and variable names follow Python naming conventions.
* [x] All function parameters and return values are annotated with Python [type hints](https://docs.python.org/3/library/typing.html).
* [ ] All functions have [doctests](https://docs.python.org/3/library/doctest.html) that pass the automated testing.
* [x] All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
* [ ] If this pull request resolves one or more open issues then the commit message contains `Fixes: #{$ISSUE_NO}`.
