Httpx support

[IlyaSukhanov]

Add support for httpx

As httpx strives for close compatibility with requests few changes needed to be made in curlify

• .lower() on headers (headers in http are case insensitive and httpx / requests use different cases.
• httpx does not have a .body but contents can be accessed via .read() instead, curlify will now use either or.
• httpx stores url in as URL object so we stringify it.

Majority of the changes are in tests.

• all tests are mocked out so there is no hitting of external services
• payload tests are more relaxed. Testing for approximate existence of flags rather than exact string match. This is a workaround to deal with various discordant optional http headers.

Unrelated changes include a makefile to aide with testing and update of testing requirements.

[tdonovic]

I know this library has a long time since anything has been merged, but I would love to use this change!