Add requirements.txt and correct the dependency list

Add a requirements file (pyscard, opencv-python, openpyxl) so users can
pip install -r requirements.txt. Remove the incorrect cryptography
dependency from the README - all crypto uses the Python standard library.

requirements.txt

@@ -0,0 +1,9 @@
+# otp-import runtime dependencies (Python 3.9+)
+# Install with:  pip install -r requirements.txt
+#
+# All cryptography (PBKDF2, HMAC, Base32) uses the Python standard library,
+# so there is no separate crypto dependency.
+
+pyscard>=2.0         # PC/SC smartcard access (apex_otp_import.py)
+opencv-python>=4.5   # QR-code decoding (qr_to_sheet.py)
+openpyxl>=3.0        # reading legacy .xlsx input (apex_otp_import.py, tests)