Remove PDK build targets from Makefile

[donn]

As discussed--

• pdk can now only be fetched with volare -- users who want to build their own PDK should know what they're doing
• volare, rst_include now share a venv
• venv no longer needlessly recreated every single time volare is installed
• venv added to gitignore
• check-python reimplemented

[d-m-bailey]

@donn thanks for the quick response.

Some comments.
1.

	@$(PYTHON_BIN) -c "import sys; assert sys.version_info >= (3, 6), 'Python version less than 3.6'"
	@echo "Python >=3.8 found."

Python 3.6 and 3.7 will pass the assertion, but the message will be >= 3.8, won't it?

2. Can we keep the deprecated targets, but add messages to the ones to give the user some idea of what changed?
   For example, with skywater-pdk

skywater-pdk: 
	@echo "The skywater-pdk target has been deprecated. Please use make pdk-with-volare."
	exit 1

3. This is pedantic, but since the check-env dependency is included in pdk-with-volare, it's probably not needed on the pdk target too.

4. Since there are changes to the README.rst target, can you go ahead and make the change to remove openlane/README.rst too?

[d-m-bailey]

@DavidRLindley @donn Since this was just merged, I'll make the above changes along with the my changes later.

The python message should be, >=3.6 right?

[donn]

@d-m-bailey Yes, oops. Thank you.

I initially wanted it to make it 3.8 but I just know someone on CentOS 7 will manifest from thin air.

[d-m-bailey]

@donn There may be a problem with the pdk-with-volare dependency requirements.txt.

It works when executed from caravel directory, but when executed from the caravel_user_project directory, I get

make[1]: *** No rule to make target 'requirements.txt', needed by 'venv/manifest.txt'.  Stop.

Would changing

venv/manifest.txt: ./requirements.txt

to

venv/manifest.txt: $(CARAVEL_ROOT)/requirements.txt

be sufficient?

[donn]

I believe so.

[d-m-bailey]

@donn Thanks. There's a place in the recipe that needs to be changed also. I'll make the change.