Undefined name: import warnings for lines 120 and 123

[cclauss]

flake8 testing of https://github.com/vinayak-mehta/present on Python 3.9.0rc2+

$ flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics

./present/slide.py:119:13: F821 undefined name 'warnings'
            warnings.warn("Codio speed < 1, setting it to 1")
            ^
./present/slide.py:122:13: F821 undefined name 'warnings'
            warnings.warn("Codio speed > 10, setting it to 10")
            ^
2     F821 undefined name 'warnings'
2

https://flake8.pycqa.org/en/latest/user/error-codes.html

On the flake8 test selection, this PR does not focus on "style violations" (the majority of flake8 error codes that psf/black can autocorrect). Instead, these tests are focus on runtime safety and correctness:

• E9 tests are about Python syntax errors usually raised because flake8 can not build an Abstract Syntax Tree (AST). Often these issues are a sign of unused code or code that has not been ported to Python 3. These would be compile-time errors in a compiled language but in a dynamic language like Python, they result in the script halting/crashing on the user.
• F63 tests are usually about the confusion between identity and equality in Python. Use ==/!= to compare str, bytes, and int literals is the classic case. These are areas where a == b is True but a is b is False (or vice versa). Python >= 3.8 will raise SyntaxWarnings on these instances.
• F7 tests logic errors and syntax errors in type hints
• F82 tests are almost always undefined names which are usually a sign of a typo, missing imports, or code that has not been ported to Python 3. These also would be compile-time errors in a compiled language but in Python, a NameError is raised which will halt/crash the script on the user.