gh-99418: Make urllib.parse.urlparse enforce that a scheme must begin with an alphabetical ASCII character. (GH-99421)

Prevent urllib.parse.urlparse from accepting schemes that don't begin with an alphabetical ASCII character.

RFC 3986 defines a scheme like this: `scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )`
RFC 2234 defines an ALPHA like this: `ALPHA = %x41-5A / %x61-7A`

The WHATWG URL spec defines a scheme like this:
`"A URL-scheme string must be one ASCII alpha, followed by zero or more of ASCII alphanumeric, U+002B (+), U+002D (-), and U+002E (.)."`
(cherry picked from commit 439b9cfaf43080e91c4ad69f312f21fa098befc7)

Co-authored-by: Ben Kallus <49924171+kenballus@users.noreply.github.com>