Add Indian Financial System Code (IFSC) bank branch code validation

[yashbudhia]

This adds stdnum.in_.ifsc for the Indian Financial System Code, the bank branch identifier the Reserve Bank of India assigns to every branch that participates in NEFT, RTGS and IMPS. It is required for every domestic electronic funds transfer in India and is also used to route UPI payments, so it shows up in practically every Indian payments, lending and KYC system. The in_ package already covers PAN, GSTIN, Aadhaar, EPIC and VID; IFSC is the missing banking one.

Format

• 11 characters: 4 letter bank code, a fifth character that is reserved and always 0, then a 6 character branch code.
• The branch code is usually numeric but may contain letters, which is common for cooperative and other sub-member banks that transact through a sponsor bank (e.g. HDFC0CABBLM).
• There is no check digit, so this is structure validation in the same spirit as stdnum.bic.

Sources

• https://en.wikipedia.org/wiki/Indian_Financial_System_Code
• RBI's IFSC/MICR list: https://www.rbi.org.in/Scripts/IFSCMICRDetails.aspx
• Razorpay's openly maintained dataset and lookup service derived from the RBI list: https://github.com/razorpay/ifsc and https://ifsc.razorpay.com/

What is included

• compact(), validate(), is_valid() and info() (returns bank_code and branch_code), following the conventions of the existing in_ modules.
• tests/test_in_ifsc.doctest with 60 real codes taken from the Razorpay dataset, chosen to span many different banks and both numeric and alphanumeric branch codes, plus corner cases for length, the reserved fifth character, bad characters and normalisation.
• The dataset's 1,296 representative codes were checked while writing this: all are 11 characters and every fifth character is 0, so InvalidComponent for a non-zero fifth character is justified.

Per CONTRIBUTING I have not touched NEWS, README or docs. pytest, flake8 and scripts/check_headers.py pass on the new files; mypy reports only the pre-existing zeep import notes that pan.py also reports.